You can replace "what would have happened" reasoning with "what will happen" reasoning. And "what will happen" reasoning is done by any model that can make predictions i.e. any scientific model. I don't understand why you think that "exactly the same or exactly opposite" matters for this proof. There is another short proof, but unfortunately it is behind paywall https://journals.aps.org/pra/abstract/10.1103/PhysRevA.47.R747 It assumes very little but it's locality definition is somewhat specific.