My theory regarding timelines and lifestream is very simple.
One thing we know for sure is that Zack jumps like a kangaroo between one timeline and another.
What made me think is that every time he makes a choice (Biggs, Hojo) he is promptly surrounded by Shinra soldiers;
this led me to only one conclusion and that is that in any case fate repeats itself and he dies.
The only time this doesn't happen or at least not like this is when it's outside the church. Here he is made to disappear from the dimension by Uncle Sephiroth and then guided to Cloud by Aerith probably.
All these timelines and all these worlds have one thing in common and that is that they are worlds, according to Uncle Sephiroth, that have agreed to collapse (meteors).
All worlds/timelines return to the lifestream (always Uncle Sephiroth docet) and return to the moment of destruction.
Obviously the only one who seems to be able to travel between timelines is Aerith (v. dream) who according to my theory is nothing more than a version of her already made into a kebab and who reunited with the lifestream to defeat Sephiroth.
Long story short:
timeline are nothing more than a narrative method for:
1) Zack and Aerith reunion
2) Justify Cloud's visions in ACC
So even if I was partly right, all these ultra-dimensional things will collapse on themselves to allow the only true reality to continue to exist and at the same time it would allow for a happy, and at the same time bittersweet, ending for everyone.