Do you think I actually committed that mistake?

I'm claiming that the substitution principle isn't so much a model as a (non-information-adding) rephrasing of the existence of heuristics, so there isn't much of a "mistake" to be made - the rephrasing isn't actually wrong, just unhelpful.

Unless of course you actually think the new question is explicitly represented in the brain in a similar way that a question read or heard would be, in which case I think you've made that mistake in every single one of your examples, unless you have data to back up that assumption.

But it being possible to misapply something doesn't make the thing itself bad.

Being possible to easily misapply an explanation does make it bad, because that means it's not anticipation-constraining.

The substitution principle provides us with the (obvious in retrospect) anticipation that "if your brain instantly returns an answer to a difficult problem, the answer is most likely a simplified one"

This is exactly what I'd expect the moment after learning of the existence of natural heuristics. If the brain is answering a question in less time than I'd expect it to take to calculate/retrieve the answer, obviously it's doing something else. What this post seems to be trying to add is that "doing something else" can be refined to "answering a different question" - but since the brain is providing output of type Answer, any output will be "answering a different question", so it's not actually a refinement.

It's possible you just wanted to explicitly state a principle that happened to be implicitly obvious to me, in which case we have no disagreement. But the length of the post and the fact that you bothered to cite Kahneman seem to me to indicate that you're trying to say something more substantial, in which case I've missed it.