The problem is that stage 2 is impossible. The thing that makes the question unanswerable is missing information, or a genuine misunderstanding on the part of the asker. It's very rare for phrasing to be the problem.

And it is terribly annoying when someone changes the question and it no longer leads towards an answer that is useful for the asker.

Yes, that's why I thought OP's idea of explicitly separating the derived "better" questions interesting, instead of allowing anyone to edit the original question and steer the discussion in a different direction. This is an interesting format that leaves room for multiple directions to explore… which SO does not want to do as they insist so much on focus.

Sometimes the asker is missing a concept, and it's not always impossible to comment about it and rephrase the question to make it answerable & more useful to keep for posterity. It's far from the majority of questions, but not awfully rare either. Some of those are worth salvaging.

If you understand enough of the question to rephrase it, you probably understand it well enough to answer it too. Adding a middle step just gets in the way. I've answered questions like that, and it's gratifying when it turns out your understanding was correct.

I upvoted because I find the idea of an explicit third stage interesting. But SO does offer the ability for experts to edit questions already, in order to keep the simple Q&A format with two stages, so it's "by design". I'm not arguing which design is better... I'd actually like to see your idea implemented :)