Can a Constitutional anon enlighten me here?

Since it seems a given to a severely contested election according to the Q [and Hell on earth is about to begin via BLM/Antifa], the normal ladder of succession is:

1) POTUS

2) VPOTUS

3) SOTH

If all are up for reelection, and that election is contested, who then come 1/21, can legally step up to the plate if the situation has not been settled?