Thanks @mikedibaggio ! I might or might not take this hook any further, as it was just a mechanism to explain why Merwin is losing his job. So feel free to use the idea or expand on it.
Yeah, the philosophical and causality hoops. But this story is about traveling through the multiverse, so I think I can just sidestep causality problems by positing that past and future don't exist as such, but only as a subset of an infinity of parallel nows.
Yes, that's much more palatable and, I think, more interesting as it allows you more freedom to play with the same character in different forms. That is typically how Marvel solves the problem of various time travel adventures altering history (creating a splinter timeline). Or used to, before their continuity devolved into a never-ending stream of greater or lesser retcons.