I don't see the "Kingmaker" scenario working out.
The whole reason Biden became the nominee in the first place was because all of the other candidates were unacceptable to one faction or the other within the Democratic Party. Biden had the demeanor of a moderate but the policies of a progressive, so he was more or less acceptable to the party as a whole.
So I really can't see them all just hopping on board some other candidate without even a vote. Maybe Michelle Obama, but I still don't think she's interested in the job.