Interesting to think about. Biden is not President yet. Electors have not been appointed. The electors have not given their votes. He has not taken the oath of office.
Timing is critical here. If Biden hit his head and died instead of sprained his ankle, then it seems to me that this falls squarely on the electors and their selection. Succession does not matter, so none of this goes to the VP stuff, then Speaker of the House. It makes no sense to seat an elector for a dead man. One the other hand, I am sure Blue States would appoint electors for Harris.
Read Jeff Greenfield’s The People’s Choice. He covers a situation eerily similar to ours if Biden were to leave us before Dec 14. Of course he comes with liberal bias but the premise is real.