Nope, new congress is sworn in before electoral votes are counted. Republicans have both house and senate. Even if trump dies, electoral votes are still getting certified. (The costitution doesn’t say you can’t vote for dead people, besides the voting already happened, they are just counting it), and vance as the VP elect is sworn in as president on Jan 20 according to the 25th amendment. There is no ambiguity.

If vance also dies, then Mike Johnson become acting president for the rest of the term.

Even if democrats had one chamber of congress, it doesn’t matter. Both chambers are needed to vote to reject the votes, if either doesn’t reject, the electoral votes are still counted. The Electoral Count Act is now updated to clarify that the VP (and probably the speaker too) has no authority to reject votes, their role is merely ceremonial. The conservative supreme court will definitely make sure to help the candidates on their side.