The Constitution states that federal elections must be held on the same date, and we settled on our current date in 1841 (I think). This is why a Florida revote was constitutionally impossible in 2000. I even question if early voting is constitutional, but that's for the courts to decide if a challenge ever arises.
As to whether Hillary is replaced because she dies, becomes a vegetable due to a stroke, or leaves for other reasons of health, the legal situation is the same even if the political situations are different. Because of the wording of the Constitution, I have a hard time seeing how any body other than a state legislature would have jurisdiction to decide matters, no matter how many scofflaw courts, like the Florida Supreme Court, chose to claim jurisdiction. If I remember correctly, the Florida legislature had a slate of Bush/Cheney electors ready to go if the Florida Supreme Court continued its obstructionist behavior by the time the Safe Harbor Date came around.
Admittedly, law is one thing and politics another, but I have problems with your scenario. From a legal perspective, it's not intuitively obvious.
I don’t know how to answer: you agree the states are supreme on this; and that any votes for a dead person ( or one kicked off the ticket) would be null and void.
That is precisely my point. Courts would refuse to ratify the “early votes” which would deny the Dems multiple thousands of votes and count only new ballots. Dems lose. Those people sue based on Bush v. Gore saying they were deprived. They are right. But the only situation is a whole NEW revote.
Despite the “elector tweak” I don’t think you can get around Bush v. Gore. All vote based on same ballot or none do.