Yes, thank you for that question.
I believe that the “main ideas” of a group are the ideas which are mentioned as the “main ideas” by that very group or the opposite ideas to those which are mentioned as “not ideas of the group” by that very group.
Now let me suppose that you agree with me. If you agree with me, then here is the evidence that the idea of Abhidhamma as not the Buddha’s original teaching is “not the idea” of the Theravada group, hence the main idea would be that Abhidhamma is the Buddha’s original teaching. Nevertheless, I think this is an important text in the Commentaries which shows why it is good to accept Abhidhamma as the Buddha’s original word.
Pali Text Society Translation Series nos. 8,9
The Expositor (Atthasalini); Translated by Pe Maung Tin, M.A; edited and revised by Mrs. Rhys Davids, D.Litt., M.A., starting at page 35; I have neglected the Pali diacritics for comfortable copying.
“Thus as rehearsed at the Council, the Abhidhamma is a Pitaka by Pitaka-classification, Khuddaka-Nikaya by Nikaya-classification, Veyyakarana by Part-classification and constitutes two or three thousand units of text by the classification of textual units. One of those bhikkhus who studied the Abhidhamma once sat in the midst of bhikkhus who knew all the five Nikayas, and quoting the text (sutta) from the Abhidhamma taught the Doctrine thus: ‘The aggregate of matter is unmoral; of the four (mental) aggregates some are moral, some immoral, and some unmoral. Ten sense-organs are unmoral; the (remaining) two sense-organs may be moral, immoral, or unmoral. Sixteen elements are unmoral; the (remaining) two elements may be moral, immoral, or unmoral. The Fact of the Origination of Ill is immoral; the Fact of the Path is moral; the Fact of Cessation is unmoral; the Fact of Ill may be moral, immoral, or unmoral. Ten controlling powers are unmoral; the controlling power of grief is immoral; the controlling power of (intellect which prompts and inspires us) - “I shall come to know the unknown” - is moral; four controlling powers may be moral or unmoral; six controlling powers may be moral, immoral or unmoral.’ A bhikkhu, seated there, asked, ‘Preacher, you quote a long text as though you were going to encircle Mount Sineru; what text is it?’ ‘Abhidhamma text, brother.’ ‘Why do you quote the Abhidhamma text? Does it not behove you to quote othertexts spoken by the Buddha?’ (Preacher) ‘Brother, by whom was the Abhidhamma taught?’ ‘Not by the Buddha.’ (Preacher) ‘But did you, brother, study the Vinaya-Pitaka?’ ‘No, brother, I did not.’ (Preacher) ‘Methinks, because you have not studied the Vinaya-Pitaka, you say so in ignorance.’ ‘I have, indeed, brother, studied some Vinaya.’ (Preacher) ‘Then that has been badly acquired. You must have been seated at one end of hte assembly and dozing. A person who leaves the world under such teachers as yourself to give the Refuge-formula, or a person who receives the full ordination under a chapter of such teachers as yoursellf, who have badly studied the Vinaya, does amiss. And why? Because of this badly “studying some Vinaya.” For it has been said by the Buddha: “If without any intention of reviling the Vinaya one were to instigate another, saying, Pray study the Suttas or Gathas or Abhidhamma first and afterwards you will learn the Vinaya - there is no offence in him.” (Again, in the Bhikkhuni Vibhanga: “A bhikkhuni is guilty of a minor offence) if she questions on the Abhidhamma or Vinaya after getting permission (to question) on the Suttanta, or on the Suttanta or Vinaya after getting permission (to question) on the Abhidhamma, or on the Suttanta or Abhidhamma after getting permission (to question) on the Vinaya.” But you do not know even that much.’ With so much refutation was the heretic put down.”
The word for “heretic” in the Pali original is “paravādī”, literally “(follower) of others’ ideas.” In other words, somebody who doesn’t accept the main teachings and instead accepts a different teachings. The text continues -
The Mahagosinga Sutta is even a stronger authority (to show that the Abhidhamma is the Buddha’s word). For therein when Sariputta, the Generalissimo of the Law, approached the Teacher to inform him of the reciprocal questions and answers that took place between Mahamoggallana and himself, and told how the former had answered, (the Master said) 'Brother Sariputta, in the religion the talk of two bhikkhus on the Abhidhamma, each asking and answering the other without faltering, is in accord with the Dhamma. Now such a bhikkhu, brother Sariputta, might enhance the beauty of the Gosinga Sala Forest. The Teacher, far from saying that bhikkhus, who knew Abhidhamma, were outside his religion, lifted his drum-like neck and filling (with breath) his mouth, fraught as the full-moon with blessings, emitted his godlike voice congratulating Moggallana thus: ‘Well done, well done, Sariputta! One should answer rightly as Moggallana has done; Moggallana is indeed a preacher of the Dhamma.’ ANd tradition has it that those bhikkhus only who know Abhidhamma are true preachers of the Dhamma; the rest, though they speak on the Dhamma, are not preachers thereof. And why? They, in speaking on the DHamma, confuse the different kinds of Kamma and of its results, the distinction between mind and matter, and the different kinds of states. The students of Abhidhamma do not thus get confused; hence a bhikkhu who knows Abhidhamma, whether he preaches the Dhamma or not, will be able to answer questions whenever asked. He alone, therefore, is a true preacher of the Dhamma. To this the Teacher referred when he approving said, ‘Moggallana has well replied to questions.’ He who prohibits (the teaching of ) Abhidhamma gives a blow to the Wheel of the Conqueror, denies omniscience, subverts the Teacher’s knowledge full of confidence, deceives the audience, obstructs the path of the Ariyas, manifests himself as advocating one of the eighteen causes of dissension in the ORder, is capable of doing acts for which the doer is liable to be excommunicated, or admonished, or scorned (by the Order), and should be dismissed after the particular act of excommunication, admonition, or scorn, and reduced to living on scraps of food.
I am curious whether @Viveka would suggest here that the author of the Commentaries also uses “very emotive language” and suggest him “a bit restraint and dispassion.” 
The text further continues. The word for heretic here is not included the original Pali text and was apparently added by the translator for easier reading -
"But if the heretic should say, had Abhidhamma been taught by the Buddha, there would have been an introduction prefatory to it, just as in many thousands of the Suttas the preface generally runs as, ‘One day the Blessed One was staying in Rajagaha,’ etc., he should be contradicted thus: ‘The Jataka, Suttanipata, Dhammapada, and so on, have no such introductions, and yet they were spoken by the Buddha.’ Furthermore he should be told, ‘O wise one, this Abhidhamma is the province of the Buddhas, not of others; the descent of the Buddhas, their birth, their attainment of perfect wisdom, their turning of the Wheel of the Law, their performance of the Twin Miracle, their visit to the devas, their preaching in the deva-world, and their descent therefrom are all manifest. It would be unreasonable to steal the Treasure-elephant, or horse of the universal Monarch and yoke it to a cart and drive about, or the Treasure-Wheel and fix it to a hay cart and drive about, or to use the Treasure-jewel capable of shedding light to the distance of a yojana by putting it in a cotton basket - and why? Because they are royal property. Even so Abhidhamma is not the province of others; it is the province of the Buddhas only. Such a discourse as the Abhidhamma can be taught by them only; for their descent is manifest … likewise their return from the deva-world. There is, O wise one, no need for an introduction to Abhidhamma.’ When this is so stated, the heterodox opponent would be unable to adduce an illustration in support of his cause.
Here “heterodox” is in the Pali text “paravādinā”, i.e., “by the (follower) of another one’s teachings.”
The Elder Tissabhuti, resident at the Central Park, wishing to show that the place of the Great ENlightenment is an introduction to Abhidhamma, quoted the Padesaviharasutta - ‘Bhikkhus, by whatever mode of life I lived after I first attained Buddhahood, I have [these two weeks] lived by that mode of living.’ This he expanded: There are ten positions: of the aggregates, the field of sense, the elements, the Truths, the controlling powers, the causal signs, applications of mindfulness, jhana, mind, and states. Of these the Teacher at the foot of the great Wisdom Tree intuited the five aggregates fully; for three months he lived only by way of the aggregate of feeling. He intuited the twelve sense-organs and the eighteen elements fully; for three months he lived only, by way of feeling, in the field and in the element of mental presentations. He intuited the four Truths fully; for three months he lived only by way of feeling in the Truth of Ill. He intuited the twenty-two controlling faculties fully; for three months he lived only by way of the five emotional indriyas (Fn: Vibhanga 123). He fully intuited the chain of the causal genesis; for three months he lived by way of feeling with touch as its cause. He intuited the four applications in mindfulness fully; for three months he lived only by way of feeling to which mindfulness was intensely applied. He intuited the four Jhanas fully; for three months he lived only by way of feeling among the factors of Jhana. He intuited mindfully; for three months he lived by way of feeling mind only. He intuited (other) states fully; for three months he lived only by way of (one or other of) the triplet of feeling. Thus the Elder set forth an introduction to Abhidhamma by means of the Padesaviharasutta.
In the following portion, “heterodox” is in original Pali “paravādī”, as we have seen already two times above.
"The Elder Sumanadeva, resident in a village, while translating the Scriptures at the base of the Brazen Palace, thought: ‘This heterodox believer, who does not know the introduction (nidana) to Abhidhamma, is just like one crying (helpless) with uplifted arms in the forest, or like one who has filed a lawsuit without witness.’ And in order to show the introduction, he said, 'At one time the Blessed One lived among the gods on the Pandukambala rock at the foot of the Paricchattaka tree in Tavatimsa. Then the Blessed One taught Abhidhamma to the Tavatimsa. Then the Blessed One taught Abhidhamma to the Tavatimsa gods thus: ‘moral, immoral, and unmoral states of consciousness.’ etc.
Whereas in the Sutta discourses there is but one introduction, in Abhidhamma there are two: one on the Career and its Goal, and one on the teaching. Of these the former comprises the events from the time of Dipankara of the Ten Powers up to the time of attaining the throne under the Wisdom Tree; the latter comprises the events between the last mentioned and the time of turning the Wheel of the Dhamma. Thus for proficiency in the introduction to Abhidhamma, which has both of these, the following questions should be asked: 1. From which source has this Abhidhamma originated? 2. Where has it matured? 3. Where, 4. when, and 5. by whom was it mastered? 6. Where, 7. when, and 8. by whom was it studied? 9. Where, 10. for whose benefit, and 11. for what purpose was it taught? 12. By whom was it accepted? 13. Who are learning it? 14. Who have learnt it? 15. Who knows it by heart? 16. Whose word is it? And 17. by whom has it been handed down?
The reply to these is: 1. Faith which urges to enlightenment was the source. 2. In the five hundred and fifty Jatakas. 3. At the foot of the Wisdom Tree. 4. On the full-moonday of Visakha. 5. By the omniscient Buddha. 6. At the foot of the Wisdom Tree. 7. During the seven days spent at the Jewel House. 8. By the omniscient Buddha. 9. Among the Tavatimsa devas. 10. Of the devas. 11. For release from the four Floods. 12. By the devas. 13. Probationers and good worldlings. 14. Saints free from the Intoxicants. 15. Those who lay it to heart. 16. Of the Blessed the Arahant, the Buddha Supreme. 17. By the unbroken line of teachers. It was conveyed up till the time of the third Council by the Elders Sariputta, Bhaddaji, Sobhita, Piyajali, Piyapala, Piyadassi, Kosiyaputta, Siggava, Sandeha, Moggaliputta, Visudatta, Dhammiya, Dasaka, Sonaka, Revata, and others. After that, it was conveyed by a succession of their pupils."
I like reading all . Lastly I did read a lot of the Indian sources. There is obvious disagreement in tradition.