I would put Biden first.
I think it would be easiest to get the courts to require that Biden be allowed on state ballots notwithstanding the fact that many deadlines for ballot access are already past, on the basis that he is the sitting Vice President and already well known nationally.
Even so, I would guess they would need to replace her before the three nationally-televised debates to get a favorable ruling.
If only the Dems had one more Supreme Court justice, they could dispense with this whole pesky election thing and just interpret the Constitution to allow substitution of a self-perpetuating Politburo.
I think Kaine has the best claim. He’s already on the ticket in all 50 states.