So far as the Bill Clinton option goes, I can't see how that 22nd Amendment would be relevant, and would make him ineligible to be elected VP.

On the face of it, it just means he can't be "elected" president again. But a vice president who becomes president because a vacancy arises hasn't in fact been "elected" president, but has succeeded to it by virtue of being next in line next in line.

And the Twelfth Amendment doesn't alter matters - "no person constitutionally ineligible to the office of President shall be eligible to that of Vice-President of the United States." But being ineligible to be elected President is not the same as being ineligible to actually be president through some other mechanism which does'nt involve being elleced to the position.

Of course the Supreme Court might be quite capable of deciding that the words didn't mean what they said and meant something else, and it wouldn't be the first time that's happened. But until and unless that has happened the clear meaning of the amendments is the one that holds.

But I think it's fairly safe to anticipate that the situation isn't going to arise. Not even if Mrs Clinton were the Democratic presidential nominee.