kirk5a wrote:But where do we find the evidence for all those conclusions?
Nothing conclusive, of course, as has been mentioned already. Reasoned acceptance of a view still turns out in one of two ways, neh?
Nevertheless, the garudhammas have been discussed thoroughly, here and elsewhere, as being late. The mundane-supramundane distinction is largely seen as part of the Nikaya-Abhidhamma transition, if not solidly housed within the Abhidhamma stratum, known to post-date the Buddha. The fifth jhana factor is part of the same layer, as is the cessation attainment.
The arupas are necessarily earlier than talk of the cessation attainment, for example, and some form of the arupas seem to have been practiced before the Bodhisatta was born, while the jhanas the Buddha taught do not seem to have been. Certainly the awakening factors were not, which amounts to the same thing.
Yet it's all in the Buddha's mouth - a thoroughly ahistorical result.