That's totally inaccurate. The fact that future itself, in all its possible forms, is speculative makes it one giant composition of variables. And the very definition of "variable" means multiple parallel scenarios--And in the end, that would mean "alternate".

Yes. We have one set destiny. But the fact of the matter is: We do not know it. We can only see things from our own POV and not an omnicient all knowing one, in which case, we have to measure odds and theory.

Unless there was some sort of dictator of time, tangible or not, in the JLU that spouts out prophesies with perfect accuracy, which, in and of themselves, lead people down that specified path, I don't see your arguments as having a lot of credence in the case of the JLU continuum. Even then, your context would have to remain in the vicinity of that dictator's governing hand.