Various Time-Travel Tropes that *could* be in effect... I am not responsible for the wiki walks that might ensue from these... Really, the entire
Time Travel Tropes list is a good thing to overview...
Delayed Ripple Effect - Changes made to the Past take time to "catch up" to the Present.
Ripple Effect-Proof Memory - The people responsible for changing the Past remember what happened "first time around", even though that wouldn't have happened to them in the Present.
Ripple Effect Indicator - An object from the Present/Future that reflects the changes made by changing the Past.
Caught in the Ripple - Something new exists (between installments/episodes) that everyone in-universe treats as having always been the way things were in previous installments/episodes.
Tricked Out Time - Faking a stable time loop by changing the Past in such a way that it *looks* like the original event occurred, when instead, something else happened.
Close-Enough Timeline - The Past gets changed, but it's close enough to what happened the first time, no one bothers changing it.
In Spite of a Nail - The Past gets changed, but the change gets "smoothed out" so the new changes don't have an effect on the Present.
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Delayed Ripple Effect is *probably* what is going on after the Singularity happens. With a hefty dose off Ripple Effect-Proof Memory on the side of AVALANCHE. They conveniently end up not being in Midgar where whatever changes could have been made when Zack didn't die would probably be a lot more obvious.
Stamp is the obvious Ripple Effect Indicator. There are a few others sprinkled around earlier that hint at time *already* being changed before the game even starts. But everyone is Caught in the Ripple and isn't noticing them (yet).
Tricked Out Time is probably what is going on with the Singularity and Zack not dying... but Tifa still finding Cloud (and the Buster Sword) like she did in the OG. It's close enough to the OG that the major differences aren't obvious (yet).
Close-Enough Timeline is what Aerith is more than likely aiming for. Sephrioth is aiming for
Make Wrong What Once Went Right.
In Spite of a Nail is what it seems most *players* are hoping Part 2 will be...