Plan B was a Clinton Obama ticket. The Maxwell arrest casts doubt on that now. They may be stuck with Biden now. For what ever reason they won’t run Sanders, never did understand why.
I agree that they may feel they're stuck with Biden. I my opinion, if the DNC did decide to pull a switch, they wouldn't replace Biden with Hillary. Hillary is already damaged goods. As you've stated, the Maxwell arrest complicates things further. Biden has already backed himself into a corner by all but guaranteeing that his VP will be a Black female. If Biden is replaced, there will be a huge backlash from Black Democrats if they pick a White replacement. Also, the Bernie Bros will demand that the nomination should now go to Sanders.
The horse is already out of the barn, but they didn't want Bernie because he had REAL enthusiasm from his crew. The head of the ticket becomes the leader of the party. The Rats wanted a "centrist" like Bill or Hill or what Obama pretended to be. Bernie at the head of the party would turn them into a full blown communist party.
Course, Omar, AOC and a couple of others have already dragged the party there. Now, the Rats are stuck with a pile of old corpses or young communists. That's all that's left in the party.