I feel like I have to reiterate some stuff already:
If BB3 doesn't happen, Bale is still contracted for a third movie (which could also be an ensemble pic). If Nolan declines to return for a third, Warners can easily use that third film option on Bale for JL:M if they're reworking it into the Nolanverse continuity.
Bale won't like it, but if he doesn't do it WB will likely sue him. Unless there was a contract proviso that he can only work with Nolan on a third Bat-film...
Contracts aren't worth the paper they are written on, if Bale doesn't want to come back, he won't, there's always a loophole.