For (b), imagine a set S of all possible universes. Then the two universes that agree perfectly up to the chosen event and then diverge are certainly both in S. And since they are non-interacting, there's no problem with energy conservation.
FWIW, the existence of infinite universes to justify calculations also strikes me as deeply unsatisfying...
I don’t have any trouble with the two universes, conceptually. I have trouble with the infinite others. I don’t understand the idea that the two convenient universes are represented by the wave function, but the infinite inconvenient ones are not.
But maybe the answer is that all universes are somehow represented by the wave function and I just don’t understand the implications. This might indeed be the case.
I tend to agree that this is deeply unsatisfying, regardless. Many worlds is interesting but also feels extremely far fetched. It honestly feels a little bit like “we don’t understand this yet, must be magic”.
We’ve invoked an extremely complicated phenomenon to answer a gap in our understanding. And maybe it’s true, but that’s what ancient astronomers thought about the wild paths planets took, too. “The math works out with circles in circles. Occam’s razor, amiright Ptolemy?”
FWIW, the existence of infinite universes to justify calculations also strikes me as deeply unsatisfying...