If it’s possible to use decision theory in a deterministic universe, then MWI doesnt make things worse except by removing refraining. However, the role of decision theory in a deterministic universe is pretty unclear, since you can’t freely decide to use it to make a better decision than the one you would have made anyway.
MWI is deterministic over the multiverse, not per-universe.
Yes, and it still precludes free choice, like single universe determinism, as well as precluding refraining