I assume your question is hypothetical.
First, the facts would have to be indisputable, that is, everyone would agree that there had been widespread fraud. This would precipitate a constitutional crisis as there is no procedure for what to do in this case. None of your solutions exist in the Constitution, so we would default to existing law.
Assuming only the presidency was affected, once the current president’s term expired, the Speaker of the House would become president. I would assume that their first order of business would be to come up with a way to choose the next president.
If the entire election was agreed to be invalid, then there would be no Speaker as the terms of every House member would also have expired. All we would have are the 66 senators whose terms don’t expire until 2022 or 2024. Per the Presidential Succession Act, the President Pro Tempore of the Senate would then be the president.
Since Democrats would have the majority in that case, presumably the née Senate would select the longest serving Democratic senator, which would be Patrick Leahy of Vermont. He would then take steps to reconstitute the government.
Interesting question. I learned the above from electoral-vote.com.
