Thank you for this important question! 
I will try to give a detailed answer.
First, we may note that the EBTs view ethical conduct as an expression of compassion. Support of this position comes from the Madhyama-āgama parallel to the Karajakāya-sutta, which states that one who kills living beings lacks compassion. This implies that one who abstains from killing living beings thereby acts with compassion. Similarly, refraining from other unwholesome behavior can be an expression of the compassionate wish for others to be free from affliction.
“What are the three types of intentionally performed bodily deeds that are unwholesome, that result in the experience of dukkha and that have dukkha as their fruit? Killing living beings is reckoned the first. Someone is supremely bad and blood-thirsty, having the wish to injure and being without compassion for living beings, including insects. (MĀ 15; full translation below)
Similarly, the Sāleyyaka-sutta states that:
And how is principled and moral conduct threefold by way of body? It’s when a certain person gives up killing living creatures. They renounce the rod and the sword. They’re scrupulous and kind, living full of compassion for all living beings. (MN 41)
Right speech can also be an expression of compassion according to the Abhayarājakumāra-sutta:
The Realized One knows the right time to speak so as to explain what he knows to be true, substantive, and beneficial, and which is liked by others. Why is that? Because the Realized One has compassion for sentient beings. (MN 58)
Returning to the Madhyama-āgama parallel to the Karajakāya-sutta (MĀ 15), we learn that the practice of the brahmavihāras needs to be grounded in ethical conduct. Given the importance of this sutta, I have included the full translation below.
Click here to read MĀ 15 (no translation is available at SuttaCentral)
Thus have I heard. At one time the Buddha was dwelling at Sāvatthī, staying in Jeta’s Grove, Anāthapiṇḍika’s Park. At that time, the Blessed One told the mendicants: “If someone performs deeds intentionally, I say that they will inevitably have to experience their fruits, either experiencing them in this life or experiencing them in a later life. If someone performs deeds unintentionally, I say that they will not necessarily have to experience their fruits.
“Herein, three are the types of intentionally performed bodily deeds that are unwholesome, that result in the experience of dukkha and that have dukkha as their fruit; four are the types of verbal deeds and three are the types of mental deeds that are unwholesome, that result in the experience of dukkha and that have dukkha as their fruit.
“What are the three types of intentionally performed bodily deeds that are unwholesome, that result in the experience of dukkha and that have dukkha as their fruit? Killing living beings is reckoned the first. Someone is supremely bad and blood-thirsty, having the wish to injure and being without compassion for living beings, including insects.
“Taking what is not given is reckoned the second. Out of attachment they take the possessions of others with the intention of stealing.
“Sexual misconduct is reckoned the third. They have intercourse with a woman that is protected by her father, or protected by her mother, or protected by both parents, or protected by her sister, or protected by her brother, or protected by her parents-in-law, or protected by relatives, or protected by the clan; or a woman protected by threat of corporal punishment, even one who has been garlanded in token of betrothal.
“These are reckoned the three types of intentionally performed bodily deeds that are unwholesome, that result in the experience of dukkha, and that have dukkha as their fruit.
“What are the four types of intentionally performed verbal deeds that are unwholesome, that result in the experience of dukkha and that have dukkha as their fruit? Speaking falsehood is reckoned the first. On being questioned in an assembly, or among family members, or in the king’s palace thus: ‘say what you know’, they claim to know what they do not know, or claim not to know what they know; claim to have seen what they have not seen, or claims not to have seen what they saw; they knowingly speak a falsehood either for their own sake or for the sake of others, or for the sake of wealth.
“Divisive speech is reckoned the second. Wishing to divide others, they tell those what they have heard from these, out of a wish to harm these, or else tells these what they have heard from those, out of a wish to harm those. Wishing to divide those who are united, and further to divide those who are already divided, they form factions, delight in the forming of factions and praise the forming of factions.
“Harsh speech is reckoned the third. They employ a type of speech that is rough and rude in tone, which sounds offensive and grates on the ear, that living beings neither enjoy nor desire, which causes others suffering and vexation, and which does not lead to calmness, speaking such type of speech.
“Frivolous talk is reckoned the fourth. They speak at the wrong time, speak what is not true, what is not meaningful, what is contrary to the Dharma, what does not lead to appeasement, and also commends issues that do not lead to appeasement. Disregarding the proper timing, they do not teach or admonish properly.
“These are reckoned the four types of intentionally performed verbal deeds that are unwholesome, that result in the experience of dukkha, and that have dukkha as their fruit.
“What are the three types of intentionally performed mental deeds that are unwholesome, that result in the experience of dukkha and that have dukkha as their fruit? Covetousness is reckoned the first. On seeing another endowed with wealth and all the necessities of life, they constantly have the wish and desire: ‘May I get it!’
“Irritation and ill will are reckoned the second. With a mind full of dislike and irritation, they have the thought: ‘May those living beings be killed, be bound, be arrested, be removed, and be banished’, having the wish that others experience infinite dukkha.
“Wrong view is reckoned the third. They have a view that is distorted, a view like this, declaring thus: ‘There is no efficacy in giving, there is no efficacy in offerings during a sacrifice, there is no efficacy in reciting hymns during a sacrifice, there are no wholesome and bad deeds, there is no result of wholesome and bad deeds, there is neither this world nor another world, there is no obligation towards one’s father and mother, in the world there are no worthy men who have reached a wholesome attainment, who are well gone and have progressed well, who by their own knowledge and experience abide in having themselves realized this world and the other world.’
“These are reckoned the three types of intentionally performed mental deeds that are unwholesome, that result in the experience of dukkha and that have dukkha as their fruit.
“The learned noble disciple leaves behind unwholesome bodily deeds and develops wholesome bodily deeds, leaves behind unwholesome verbal and mental deeds and develops wholesome verbal and mental deeds. Being endowed with diligence and virtue in this way, having accomplished purity of bodily deeds and purity of verbal and mental deeds, being free from ill will and contention, discarding sloth-and-torpor, being without restlessness or conceit, removing doubt and overcoming arrogance, with right mindfulness and right comprehension, being without bewilderment, the learned noble disciple dwells having pervaded one direction with a mind imbued with mettā, and in the same way the second, third, and fourth directions, the four intermediate directions, above and below, completely and everywhere. Being without mental shackles, resentment, ill will, or contention, with a mind imbued with mettā that is supremely vast and great, boundless and well developed, the learned noble disciple dwells having pervaded the entire world.
“Then the learned noble disciple reflects like this: ‘Formerly my mind was narrow and not well developed; now my mind has become boundless and well developed.’
“When the mind of the learned noble disciple has in this way become boundless and well developed, if because of associating with bad friends they formerly dwelled in negligence and performed unwholesome deeds, those deeds cannot lead them along, cannot defile them, and will not come back to meet them.
“Suppose there is a small boy or girl, who since birth is able to dwell in the liberation of the mind through mettā. Later on, would he or she still perform unwholesome deeds by body, speech, or mind?” The mendicants answered: “Certainly not, Blessed One.”
“Why is that? Not performing bad deeds themselves, how could bad deeds arise? Therefore, a man or woman, at home or gone forth, should constantly make an effort to develop liberation of the mind through mettā. If that man or woman, at home or gone forth, develops liberation of the mind through mettā, since when going towards the other world he or she will not take this body along, he or she will proceed just in accordance with the developed quality of their mind.
“Mendicants, you should reflect like this: ‘Formerly I was negligent and performed unwholesome deeds. Let the fruits of these be experienced entirely now, not in a later world.’ If liberation of the mind through mettā has become boundless and well developed like this, certainly non-returning will be attained, or else that which is even higher.
“Again, the learned noble disciple dwells having pervaded one direction with a mind imbued with compassion, and in the same way the second, third, and fourth directions, the four intermediate directions, above and below, completely and everywhere. Being without mental shackles, resentment, ill will, or contention, with a mind imbued with compassion that is supremely vast and great, boundless and well developed, the learned noble disciple dwells having pervaded the entire world.
“Then the learned noble disciple reflects like this: ‘Formerly my mind was narrow and not well developed; now my mind has become boundless and well developed.’
“When the mind of the learned noble disciple has in this way become boundless and well developed, if because of associating with bad friends they formerly dwelled in negligence and performed unwholesome deeds, those deeds cannot lead him along, cannot defile them, and will not come back to meet them.
“Suppose there is a small boy or girl, who since birth is able to dwell in the liberation of the mind through compassion. Later on, would he or she still perform unwholesome deeds by body, speech, or mind?” The mendicants answered: “Certainly not, Blessed One.”
“Why is that? Not performing bad deeds themselves, how could bad deeds arise? Therefore, a man or woman, at home or gone forth, should constantly make an effort to develop liberation of the mind through compassion. If that man or woman, at home or gone forth, develops liberation of the mind through compassion, since when going towards the other world he or she will not take this body along, he or she will proceed just in accordance with the developed quality of their mind.
“Mendicants, you should reflect like this: ‘Formerly I was negligent and performed unwholesome deeds. Let the fruits of these be experienced entirely now, not in a later world.’ If liberation of the mind through compassion has become boundless and well developed like this, certainly non-returning will be attained, or else that which is even higher.
“Again, the learned noble disciple dwells having pervaded one direction with a mind imbued with sympathetic joy, and in the same way the second, third, and fourth directions, the four intermediate directions, above and below, completely and everywhere. Being without mental shackles, resentment, ill will, or contention, with a mind imbued with sympathetic joy that is supremely vast and great, boundless and well developed, the learned noble disciple dwells having pervaded the entire world.
“Then the learned noble disciple reflects like this: ‘Formerly my mind was narrow and not well developed; now my mind has become boundless and well developed.’
“When the mind of the learned noble disciple has in this way become boundless and well developed, if because of associating with bad friends they formerly dwelled in negligence and performed unwholesome deeds, those deeds cannot lead him along, cannot defile them, and will not come back to meet them.
“Suppose there is a small boy or girl, who since birth is able to dwell in the liberation of the mind through sympathetic joy. Later on, would he or she still perform unwholesome deeds by body,
speech, or mind?” The mendicants answered: “Certainly not, Blessed One.”“Why is that? Not performing bad deeds themselves, how could bad deeds arise? Therefore, a man or woman, at home or gone forth, should constantly make an effort to develop liberation of the mind through sympathetic joy. If that man or woman, at home or gone forth, develops liberation of the mind through sympathetic joy, since when going towards the other world he or she will not take this body along, he or she will proceed just in accordance with the developed quality of their mind.
“Mendicants, you should reflect like this: ‘Formerly I was negligent and performed unwholesome deeds. Let the fruits of these be experienced entirely now, not in a later world.’ If liberation of the mind through sympathetic joy has become boundless and well developed like this, certainly non-returning will be attained, or else that which is even higher.
“Again, the learned noble disciple dwells having pervaded one direction with a mind imbued with equanimity, and in the same way the second, third, and fourth directions, the four intermediate directions, above and below, completely and everywhere. Being without mental shackles, resentment, ill will, or contention, with a mind that is supremely vast and great, boundless and well developed, the learned noble disciple dwells having pervaded the entire world.
“Then the learned noble disciple reflects like this: ‘Formerly my mind was narrow and not well developed; now my mind has become boundless and well developed.
“When the mind of the learned noble disciple has in this way become boundless and well developed, if because of associating with bad friends they formerly dwelled in negligence and performed unwholesome deeds, those deeds cannot lead them along, cannot defile him, and will not come back to meet them.
“Suppose there is a small boy or girl, who since birth is able to dwell in the liberation of the mind through equanimity. Later on, would he or she still perform unwholesome deeds by body, speech, or mind?” The mendicants answered: “Certainly not, Blessed One.”
“Why is that? Not performing bad deeds themselves, how could bad deeds arise? Therefore a man or woman, at home or gone forth, should constantly make an effort to develop liberation of the mind through equanimity. If that man or woman, at home or gone forth, develops liberation of the mind through equanimity, since when going towards the other world he or she will not take this body along, he or she will proceed just in accordance with the developed quality of their mind.
“Mendicants, you should reflect like this: ‘Formerly I was negligent and performed unwholesome deeds. Let the fruits of these be experienced entirely now, not in a later world.’ If liberation of the mind through equanimity has become boundless and well developed like this, certainly non-returning will be attained, or else that which is even higher.”
The Buddha spoke like this. The mendicants, who had listened to what the Buddha said, were delighted and received it respectfully.
– MĀ 15, translated by Anālayo (2015, pp. 170-6), edited for clarity and gender-inclusiveness.**
I think you are absolutely on point here, and the sutta translated above (MĀ 15) supports that. The wholesome conduct by body, speech, and mind enables the overcoming of the hindrances which block successful brahmavihāra practice. Which brings me to the question of spontaneous arising versus intentional cultivation:
As the Madhyama-āgama parallel to the Karajakāya-sutta quite explicitly talks about the cultivation of the divine abodes, it seems to me that the EBTs support the latter approach. Interestingly, the “four objects” method (oneself, a friend, a neutral person, and a hostile person) to cultivating love and compassion stems from the Visuddhimagga rather than from the EBTs (Anālayo 2019). The EBTs talk about developing meditative radiation in all directions.
More generally, ethical conduct lays the foundation for the full spiritual path:
According to the Buddha, careful observance of ethical precepts is the foundation of all higher achievements in the spiritual life. (From the summary of MN 6)
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