Alex, we can't be sure to 99.99 percent accuracy, but we can be as sure as these scholars seem to be:
"Several scholars who specialize in the field of early Buddhism have said that much of the contents of the Pali Canon (and its main teachings) can be attributed to Gautama Buddha. Richard Gombrich says that the main preachings of the Buddha (as in the Vinaya and Sutta Pitaka) are coherent and cogent, and must be the work of a single genius: the Buddha himself, not a committee of followers after his death.[19][20] Peter Harvey also affirms the authenticity of "much" of the Pali Canon.[21] A.K. Warder has stated that there is no evidence to suggest that the shared teaching of the early schools was formulated by anyone else than the Buddha and his immediate followers.[22] J.W. de Jong has said it would be "hypocritical" to assert that we can say nothing about the teachings of earliest Buddhism, arguing that "the basic ideas of Buddhism found in the canonical writings could very well have been proclaimed by him [the Buddha], transmitted and developed by his disciples and, finally, codified in fixed formulas."[23]"
Gombrich has written that based on his lifetime of scholarship, it has to be more true than not that the Dhamma was taught by Gautama. There seems to be sufficient evidence to support these strong scholars and historians in these opinions.
I believe the point, or the elephant, is the idea that the same strong scholarship is certain to a reasonable degree of scientific certainty that (most, all) the Mahayana sutras are not Buddha vacana; not the product of the oral recording by his disciples of what Gautama taught. Their authorship has been determined and dated. So, I feel you comparison, while appropriate, does not account for the elephant in the room. We can't be certain that all of the Pali Canon is vacana ( we know the Abhidhamma is a later work), but we can be as certain as Gombrich and these other scholars are, and that for me is good enough...until better evidence is brought forth, if any exists.
This is why I believe that practice and practical results is the key.
Alex, listening to Ven. Thanissaro recently, he made the point that a proper understanding of the Buddha's actual words and teachings is so important. The example he gave is meditation, or more closely, jhana, as the Buddha defined jhana. If we practice and seek practical results from a flawed strategy, or travel a road using an erroneous map, we can find ourselves in trouble. For me, understanding these early core teachings are such a big part of practice. I don't begrudge anyone using, for example, Dogen's teachings on zazen, but I feel it's important that there be an understanding that Buddha didn't teach
zazen, and didn't frame meditation the same way that later Japanese teachers did. Traveling the path using Dogen's map will likely take one to useful and interesting places, but the traveler might not get to the important destination the Buddha intended for his disciples.