Free will is not the opposite of causality but of coercion. Free will means that I have agency, based on my own internal mental states as well as the situation around me. It does not mean I can violate the laws of physics. It means I'm awake, aware, and don't have a gun pointed at me. (Or some other physical threat, etc.) An entity is said to have free will when it has internal mental states that can change its behavior.
Non-determinism would not save the notion of free will that people say is impossible. If I'm walking down the street and then, for no apparent reason, my arm flings out and smacks somebody, that's not free will. The un-caused isn't free will. The randomly produced without cause is not free will.
Multiverse theory is a (fairly abstruse) variation of quantum mechanics. It was suggested as a way to interpret certain things in QM that people had difficulty adding to their list of things they thought they understood.
It better not be required to understand anything about human behavior, since the idea only arose a little more than 60 years ago. Whatever did people do about late-night arguments over too much alcohol before that?
And, so far, nobody has managed to formulate an experimental test of multiverse theory. That is, nobody has managed to show that the existence, or non-existence, of these other universes, can be shown to make any detectable difference.