There is a procedure for this. To simplify the answer, Kamala Harris will become President on inauguration day in any of your listed scenarios, since she’s the current VP/VP-elect.
It’s theoretically possible that if Biden drops dead today, that the DNC could manage to pivot to a new contender, but there are two significant problems with that:
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It’s suuuper late in the process. They already called the primaries for Biden, so he has the delegates. If he dies, they go to his VP, so it would be up to Harris to give them to someone else at that point if anybody else were to be selected (essentially dropping out of the race herself). This is a bad move because giving delegates to someone who didn’t even primary will seem undemocratic, coronating somebody that the people did not even endorse for the ticket.
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Replacing Biden with someone else doesn’t give them much time to campaign. A big part of the election cycle is traveling to swing states to convince those people to vote for you. It’s not that those people are suddenly going to go vote for the other guy (Trump, in this case), but they’re far more likely to stay home because they won’t be energized for this newcomer who didn’t have enough time to court their votes, or to convince them to turn out for their policies/platform rather than be apathetic about the outcome of the election.
Oh look, he shaved his beard for Daddy Trump. Wouldn’t want Dear Leader to disapprove of your facial hair now would we, Jay Dee?