There are some interesting quotes here. In that linked post
on the question of 'which skandha is tathagatha' I noticed this phrase:
"Therefore, that the grasses, trees, thickets and groves are impermanent is the buddha nature; that humans and things, body and mind are impermanent — this is because they are the buddha nature.
I was going to comment at the time that I think it is incorrect to draw the conclusion from this that, therefore, 'buddha nature is impermanent'. I don't think it is true - but neither is its opposite - because I don't think it is correct to think about the subject in terms of 'duration' or 'persistence through time'. I think there is a sense in which that which is 'everlasting' is not something durable, which exists 'in itself', so to speak. It is imperishable in not being subject to change and decay, but it is not something completely separate to the realm of change and decay either - outside of time, rather than persisting through time. (This parallels the theological notion of God being both immanent and transcendent, as something both 'within' and 'beyond' worldly existence.)
The Yamaka Sutta is interesting also. I noted this phrase:
an uninstructed, run-of-the-mill person — who has no regard for noble ones, is not well-versed or disciplined in their Dhamma; who has no regard for men of integrity, is not well-versed or disciplined in their Dhamma — assumes form (the body) to be the self, or the self as possessing form, or form as in the self, or the self as in form.
The meaning seems to be, he imputes permanence to the aggregates, or 'does not discern' that the aggregates as impermanent, but takes them to be 'the self'. But they are not the self. However, the sutta also says that at the view that 'the Teaching explained by the Blessed One, a monk with no more effluents, on the break-up of the body, is annihilated, perishes, & does not exist after death' is incorrect. In fact the whole sutta is aimed at refuting this view, which is basically nihilism. But if you ask the question, 'what is it that is not
annihalated at death', this question is not directly addressed. Rather the point is that 'These five clinging-aggregates — not attached to, not clung to — lead to his long-term happiness & well-being.' In other words, 'unbinding' is 'seeing the impermanent as
impermanent' - not entertaining speculations about 'what is permanent'.
How this relates to the OP is another matter, though. If your intention is to demonstrate that 'insight into impermanence' is not
'mystical', I don't think you can do that, because it is not as if such questions are clear and obvious for all to see. As DN1 says, such matters are only 'perceivable by the wise', which implies that learning to see this, is
the process by which wisdom is acquired, and being able to perceive it constitutes the differention between the 'noble disciple' and the 'uninstructed run of the mill person'.
In the beginner's mind there are many possibilities; in the expert's mind there are few ~ Suzuki-roshi