This is very similar to time travel hypotheticals. If you travel back in time, and totally change the past, did the future you originally observed actually happ

This is very similar to time travel hypotheticals. If you travel back in time, and totally change the past, did the future you originally observed actually happen? People existed, they lived their lives, etc. And then suddenly you "reset" the universe to a time where that never happened. I would argue they did exist, and that the time travel event actually murdered them. I think of the universe as a computer, and any computation it does actually happens, actually exists in some sense. If the universe does a computation of my entire life, then deletes it, that life still happened. It "exists". I think the same is true of the famous cat experiment. The computation of both possible states of the cat has to be done by the universe. It needs to compute how the cat reacts to the poison gas, it needs to compute the pain it experiences, etc. That's a lot different than a single particle like your example. When you open the box, the universe may "collapse" to the second possibility, where the cat is still alive. But the first possibility still happened, and it may still be out there somewhere in an unobservable branch. In fact that model seems a lot more mathematically elegant, being completely deterministic and not having to deal with the weirdness of "collapse" events.