This is an interesting passage regarding the oral tradition behind the Mahayana sutras:
Even the Mahāyāna Sūtras, which were composed in writing and which frequently refer to writing  [2, 251, 255, 257], retain some oral features such as repetition [2, 273]. This shows that the oral tradition was firmly established and must have flourished for a considerable period before the texts were put in writing.
Also, Joseph Walsner’s Nāgārjuna in Context presents a good survey of a number of major theories as to the origins of the Mahāyāna in its opening sections, in addition to being a very readable and accessible resource for information about Ven Nāgārjuna and nascent Mahāyāna coming out of the post-EBT sectarian tradition (as he points out in the book as well, there is even a small deal of controversy as to whether the historical “ur-Nāgārjuna” can even be definitely said to have been a Mahāyānist for sure).
All in all, its a good book, even if it has chapters entitled things like “Parasitic Strategies of the Mahāyāna” !
Here is a snippet from the beginning of that chapter:
The European and Asian species of cuckoo are what is known as “brood parasites.” The female lays her eggs in the nests of other birds, who, in turn, raise her chicks as their own. A successful cuckoo can pass its eggs off as those of another species so that the other species will provide the labor and material resources necessary to raise the young to adulthood. The simple fact that Maháyánists were writing and copying unsanctioned scripture was potentially divisive. To alleviate tensions they would have had to convince the readers that their texts were buddhavacana, or “word of the Buddha.” Maháyánists employed a strategy similar to the cuckoo, by presenting their texts and ideas as “word of the Buddha.” If the Maháyánists could succeed in passing their sutras off as “word of the Buddha,” the host monastery would be obligated to preserve and reproduce Maháyána texts in perpetuity just as they would any other buddhavacana text. This chapter examines two strategies used by Maháyánists to evoke the authority of “word of the Buddha” for their texts.
When I say that the Mahayana sutras contain the teachings of the historical Buddha, what I mean is that I believe they convey important concepts which the Buddha taught, rather than necessarily being word-for-word accounts of historical events.
As a Mahayana Buddhist, I believe the Buddha taught the Six Paramitas of the Bodhisattva path for the attainment of full Buddhahood. As for whether the discourses of the Lotus Sutra, for example, are word-for-word recollections of such teachings, I honestly can’t say.
In his career as a Bodhisattva, the Buddha himself practiced the Paramitas for his future attainment of Buddhahood, as told in the Jataka tales.
Even in the Pali scriptures, the Buddha used various parables and similes, various forms of skillful means, to convey important truths. The Mahayana sutras, similarly, might be parables conveying what the Buddha taught, rather than word-for-word accounts.
John W. Pettit, while stating, “Mahayana has not got a strong historical claim for representing the explicit teachings of the historical Buddha”, also argues that the basic concepts of Mahayana do occur in the Pāli Canon and that this suggests that Mahayana is “not simply an accretion of fabricated doctrines” but “has a strong connection with the teachings of Buddha himself”.
Mahayana has not got a strong historical claim for representing the explicit teachings of the historical Buddha; its scriptures evince a gradual development of doctrines over several hundred years. However, the basic concepts of Mahayana, such as the bodhisattva ethic, emptiness (sunyata), and the recognition of a distinction between buddhahood and arhatship as spiritual ideals, are known from the earliest sources available in the Pali canon. This suggests that Mahayana was not simply an accretion of fabricated doctrines, as it is sometimes accused of being, but has a strong connection with the teachings of Buddha himself. https://en.wikipedia.org/wiki/Mahayana_sutras#Scholarly_views_on_historicity
Why were the Mahayana Sutras phrased as if spoken literally by the Buddha? This is a difficult question, and there is unlikely to be one answer. Partly it was just how the literary form evolved. But I suspect, given the visionary nature of many Mahayanist texts, that they often stemmed from meditation experiences; visions of the Buddha, memories of ‘teachings’ received while in samadhi. Perhaps the authors of these texts believed that the Buddha was really present to them in some sense – and this is indeed the theme of many Mahayana sutras. Or perhaps they more humbly believed that they had gained insight into the Dhamma in some direct way. https://sujato.wordpress.com/2011/10/18/is-the-lotus-sutra-authentic/
What if the Mahayana sutras really were received in samadhi? Would that make them any less true?
According to tradition, the Mahayana sutras were taught to the Bodhisattvas during the Buddha’s historical lifetime, and then kept hidden in the naga realm until the proper time. This might seem farfetched, but the nagas act as Dharma protectors in the Pali canon: http://www.accesstoinsight.org/tipitaka/kn/ud/ud.2.01.than.html
Modern physics is close to accepting that there are alternative realms of existence:
Contemporary Buddhist scholars tend to speculate the Mahayana sutras were received in states of meditative samadhi. How can we know that these meditation masters, in profound samadhi states, weren’t accessing previously unpublished sutras from the naga realm?
Or what if the Buddha, continually present in the world in a spiritual form, continued revealing new sutras after his passing to meditators in states of samadhi?
It’s important to remember that the Mahayana sutras were never meant to be interpreted literalistically, as word-for-word historical accounts. They are instead to be read for their spiritual value, as pointing to the ultimate truth, rather than constituting the ultimate truth itself.
Nagarjuna is venerated by all sects and schools of Mahayana Buddhism for his ancient commentaries on the Buddha’s teachings, so such so that he’s often referred to as the Second Buddha.
Nagarjuna is perhaps most well-known for his distinction between relative truth and Ultimate Truth, which he likened to a finger pointing at the moon:
The highest truth (paramarthasatya) is beyond words or description, i.e. beyond the reach of conceptual understanding and yet it was presented by the Buddha Shakyamuni as his teaching so that our conceptual understanding could grasp it. It is in this sense that the teaching is regarded as an ‘expedient means’ (upaya), often likened to a finger pointing to the moon. What is crucial about this metaphor is that the finger and the moon are mutually reflexive. Without the finger, the moon would not be known. Without the moon, there would be no need for the finger pointing to it. http://www.nembutsu.info/tokusuny.htm
It shouldn’t be surprising, then, that after the Buddha’s passing, additional sutras would be written as skillful means expressing important doctrinal truths, rather than as literal historical accounts.
What matters most about a Buddhist scripture is whether it’s conducive to enlightenment, not whether the Buddha historically taught it:
"As for the qualities of which you may know, ‘These qualities lead to dispassion, not to passion; to being unfettered, not to being fettered; to shedding, not to accumulating; to modesty, not to self-aggrandizement; to contentment, not to discontent; to seclusion, not to entanglement; to aroused persistence, not to laziness; to being unburdensome, not to being burdensome’: You may categorically hold, ‘This is the Dhamma, this is the Vinaya, this is the Teacher’s instruction.’" http://www.accesstoinsight.org/tipitaka/an/an08/an08.053.than.html
It’s common for Mahayana Buddhists to believe that the Mahayana sutras were either taught by the historical Buddha or formulated by enlightened teachers who came after his passing. Either way, the Mahayana sutras would be conducive to enlightenment.
Even poison, in small quantities, can be medicine.
Indeed, that particular chapter has an unfortunate title, given that we associate “parasite” with all sort of negative things, but, I found the text very “enlightening” (if we will all forgive the terrible, horrible, no good, very bad pun) despite such a title to one of the chapters.
Indeed, as the author points out, these Mahāyāna sūtrāṇi (sutras) once had to appear as “new Buddhavacana”. That was going to ruffle feathers (as it ought to) regardless of the content of the Buddhavacana in question. Walsner outlines the way in which proto-Mahāyāna and already-extant sectarian Buddhism (based on EBTs, but hardly an “Early Buddhism”) interacted, and how Mahāyānists differently conceived of the very “notion” of Buddhavacana.
Of course, he plays the “evil sangha” card, and evokes the ridiculous image of a bunch of essentially wicked Mahāyāna conspirators out to twist the Buddhavacana, rather than seeing this as a “different” (or wrong), but “believed to be true” interpretation and set of remembrances, but still gives a wonderful (IMO) and thorough summary of many leading theories as to the origins of the Mahāyāna, which seems to be the goal of your OP.
I will post some relevant excerpts shortly, making sure they are relevant to the forum.
My understanding of the Mahayana sutras is relatively simple, that they are just as conducive to enlightenment as the Pali scriptures, even if the Mahayana sutras aren’t word-for-word accounts of historical events:
Another criteria the Buddha taught to differentiate Dhamma from what was not his teaching, was that of analyzing how a particular teaching affects one’s thinking. The Gotami sutta states that anything that leads to dispassion, liberation, relinquishment, having few wishes, contentment, seclusion, arousing of energy and being easy to support are said to be the teacher’s instruction, while anything that leads to the opposite of these qualities cannot be the true teaching of the Buddha. Hence in the Early Buddhist texts, the work of hermeneutics is deeply tied with the spiritual practice and a mindful awareness of the effect our practices have on our state of mind. https://en.wikipedia.org/wiki/Buddhist_hermeneutics#Early_Buddhist_texts
It shouldn’t be surprising if the Mahayana sutras are a form of upaya or skillful means, rather than word-for-word historical accounts, especially since the Buddha himself employed skillful means in the Pali scriptures, teaching in various similes and parables to suit the understanding and capacities of his audiences:
While one might not be able to historically prove that the Buddha taught the Mahayana sutras, this isn’t the only criteria in choosing a sect or school of Buddhism.
Another criteria of choosing the right sect for oneself is evaluating the teachings of its founder or main historical commentators, to see if one finds their teachings compelling. One should also look at the lives that they lived to help assess their trustworthiness as teachers.
While I cannot prove the historical validity of the Pure Land sutras, I can nonetheless look at the Mahayana teachers throughout history, such as Nagarjuna and Vasubandhu, who’ve endorsed Pure Land practice.
I’ve also studied the life and teachings of Shinran Shonin, the founder of the Jodo Shinshu sect of Pure Land Buddhism, the largest sect of Buddhism in Japan.
Not only do I find Shinran’s explanations of Pure Land teachings compelling, he also lived a remarkable life, even enduring exile and persecution, because his teachings were considered a threat to the religious establishment of his time.
This reliance on the teachings and example of your founders isn’t unique to Mahayana schools and sects. The Thai forest tradition immediately comes to mind.