I think it'd also count as a full term in office so far as the rule against running more than twice goes. So you could run for reelection, but that isn't a "one werid trick" to getting three terms in office.
it's a two-term limit with a 10-year maximum. if the vice president gets called up 23 months before the end of the president's term, they could potentially successfully serve two and a half full terms
Also consider what happens if Trump dies at any of these stages - he's almost the same age so all else being equal he's almost as likely to die during the same period
I bet they already have the conspiracy theories prepped for when Trump dies. It would be interesting for someone to make a Reddit post asking for MAGA to share their theories that someone is trying to terminate Trump, so that when he eventually passes from natural causes, we could pull that up as relevant info to see why no one did anything beforehand to prevent the attempt or if the conspiracy that emerges wasn't stated before it happened.
The Vice President-Elect is sworn in instead. This is covered by section 3 of the 20th amendment.
Section 3.
If, at the time fixed for the beginning of the term of the President, the President-elect shall have died, the Vice President-elect shall become President. If a President shall not have been chosen before the time fixed for the beginning of his term, or if the President-elect shall have failed to qualify, then the Vice President-elect shall act as President until a President shall have qualified; and the Congress may by law provide for the case wherein neither a President-elect nor a Vice President-elect shall have qualified, declaring who shall then act as President, or the manner in which one who is to act shall be selected, and such person shall act accordingly until a President or Vice President shall have qualified.
Anything else means instant constitutional crisis, but at the same time, we'll be able hear the explosions from space as right winger minds absorb that circumstance has brought about the first female president and also the first PoC President Vice President team.
If Kamala was good at trolling she'd not nominate any white people to the cabinet so we can all enjoy fox pundits trying to find the magic combination of words that let them be mad about an all PoC cabinet without it being racist, but I'm pretty sure she's at least going to keep Buttigieg on as Transit Czar since he's been going at least hard enough to get HSR moving finally between LA and Vegas.
It doesn't work that way. First of all, Mike Johnson is the Speaker, and is next in line for the Presidency after the VP, not Jeffries. But that entire list is strictly to see who would be President if the office were suddenly vacant. The House Speaker would only take over if there is a Presidential vacancy while the office of VP is also vacant.
Should Harris becomes President due to Biden leaving office early, the office of VP would become vacant and remain so until President Harris picks a replacement, and that replacement is ratified by both houses of Congress. Sadly, in today's political climate that is not a guarantee.
Additional question. If the Vice President takes over and becomes President, do they finish the full term or are they just interim president until a new election can take place?
So the new President just picks whomever they want to be Vice President? It doesn't automatically become the Speaker of the House? Also, is there a confirmation process that the VP nominee has to complete similar to Supreme Court Justices?
The party conventions, where each parties nominee will be formalized. (But recall that the actual Democratic Nomination will be done before the convention, to meet some State deadlines).
The election itself, where voters choose which slate of electors cast EC votes.
The electoral college election that really matters.
The counting of the EC Votes
the inauguration
Once the nominations are formalized, they probably can't be undone, since there are State deadlines involved.
But always remember that people are not voting for the candidates themselves, but for a slate of electors. And while the expectation is for these electors to vote for the ticket they were pledged to, if one candidate is subject to God's Almighty Recall Vote, they appear to have the discretion to file votes differently.
Once the EC votes are cast, though, there would probably be no choice but to accept them. (Congress recently revamped the counting process to eliminate some of the shenanigans that happened last time). I expect that if either one croaks after that, they would probably just inaugurate the VP on the ticket directly.
(Edited to add: states may be able to change EC votes after the December EC election. That date is based on a "safe harbor" deadline which makes EC votes harder to challenge. So, if the winning candidate bites it on Christmas Day there's probably a way for State Legislatures to certify new EC votes before Congress counts them Jan 6, but we should expect a challenge to that.)
(It would suck if Trump won, and Biden croaked on Jan 19th. We might have our first female President, for one day.)
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:
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.
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.