You forgot 1A. Pence appoints new Vice President.
A new VP has to be confirmed by BOTH the House and the Senate, so the choice could easily be blocked by Pelosi. Even if not blocked they could insist a moderate be picked.
Mike Pompeo. Louie Gohmert. Tom Cotton.
That might be difficult with Pelosi controlling the House of Representatives. That’s why I left it out.
1a. Pence would appoint a Vice Presidential running mate.
When Vice President Spiro Agnew resigned in 1973, some of the Democrats tried to subvert the Twenty Fifth Amendment by blocking the appointment and approval of a replacement Vice-President. That would have resulted in Carl Albert (the Democrat Speaker of the House) becoming President, instead of Gerald Ford, when President Nixon resigned.