Good analysis! I’d say though that a “what if” can include non mutually exclusive situations, such as “both” and “all” situations, where reality remains true but with additions from the “what if”.
Still, I love the phrase “mutually exclusive reality” and think despite my exceptions, it’s a great start!
Or clarity (I like the train of thought you’ve inspired in me here):
“We don’t know what’s at the center of black holes yet”.
“What if the center of the black hole was an ice cream cone?”
But now I see what you mean: To make ALL whatifs true, would mean negating mutually exclusion while ALSO including mutual exclusion.
Still, that’s where someone like https://en.wikipedia.org/wiki/David_Lewis_(philosopher) can save the day.
Personally? I’d like to remove all “IF AND ONLY IF” constructs. Things get so messy with those.