Wavefunction collapse (which is actually not a thing in MWI/universal wavefunction theory) is an event or transition that moves strictly forward in time. Time

Wavefunction collapse (which is actually not a thing in MWI/universal wavefunction theory) is an event or transition that moves strictly forward in time. Time travel, if possible, would be entirely orthogonal. There is no movement between the "worlds" in "many worlds" (which is part of the reason why "many worlds" is such a misleading label). At best there's interference between them.