Here's the plan:

[returnee]

4. Won't work.

The only way the SOTH becomes president is if both Prez and VP leave simultaneously.
If Prez leaves Veep moves up and nominates a new Veep, Senate confirmation ensues. If Veep leaves, same thing, nomination/confirmation.

Harris could become President but is unlikely in the extreme. There would need to be an agreement among both parties. In my fantastical scenario, say TSF is found to have cheated and Harris really won. Let’s then say the public outcry is sufficient for a change to Harris is required. Prez resigns, Veep becomes Prez and nominates Harris as Veep after pre-agreed confirmation. Then Veep resigns and Harris becomes Prez, who then nominates Walz, confirmation. Bingo.