Anything else means instant constitutional crisis, but at the same time, we'll be able hear the explosions from space as right winger minds absorb that circumstance has brought about the first female president and also the first PoC President Vice President team.
If Kamala was good at trolling she'd not nominate any white people to the cabinet so we can all enjoy fox pundits trying to find the magic combination of words that let them be mad about an all PoC cabinet without it being racist, but I'm pretty sure she's at least going to keep Buttigieg on as Transit Czar since he's been going at least hard enough to get HSR moving finally between LA and Vegas.
It doesn't work that way. First of all, Mike Johnson is the Speaker, and is next in line for the Presidency after the VP, not Jeffries. But that entire list is strictly to see who would be President if the office were suddenly vacant. The House Speaker would only take over if there is a Presidential vacancy while the office of VP is also vacant.
Should Harris becomes President due to Biden leaving office early, the office of VP would become vacant and remain so until President Harris picks a replacement, and that replacement is ratified by both houses of Congress. Sadly, in today's political climate that is not a guarantee.