you only need 4 to hear the case, and I'm pretty sure I know who the 4 are.
they're taking the case, not to reverse the decision but to stall the cases, hopefully past the election where trump would win and moot out these cases.
and in that situation, watch them rule 9-0 after trump is sworn in that the immunity trump says he has doesn't exist.
no, but it would stop biden from doing official acts as president such as calling up seal team 6 and having them terminate with extreme prejudice certain key individuals to ensure he stays in office.