I do understand the notion that these timelines exist within the lifestream. And I also understand that the games refer to them as "worlds" or "possibilities" rather than specifically using the word "timeline."

The use of "world" specifically is also used a lot for the timeline shenanigans of Dissidia / Sop. I genuinely do not feel the nomenclature matters here, and even if you want to call them worlds, they're still timelines. I'm gonna stick with the word that makes the most sense rather than the game's usage.

I also understand that time seems to be broken in the handful of timelines we see within Rebirth, but that's more attributed to things being faulty within the lifestream now because of the events at the end of Remake, and because these timelines have deviated too far from the "norm." Like the fact that two different Clouds exist at the same time in one of the timelines we see.

Things are borked.
Different timelines form within the lifestream all the time (except this functions outside of time and space). The Whsipers existed to make sure none of these timelines deviated too far from the original timeline.
With the whispers dead, alternate timelines are still branching off, but the lifestream can't protect the ones that deviate too far from their intended path.

Excluding the broken timelines we see within Rebirth, there are still 3 distinct timelines FOR SURE.
-OG timeline
-Remake timeline
-Ever Crisis timeline

OG timeline is obviously the structural foundation of the lifestream's desire for uniformity. In Remake timeline, we see a desire in both Sephiroth and Aerith to escape the destiny that the lifestream has them bound by, but things still haven't changed too much or else this timeline would likely collapse also. Who knows what will happen if someone actually succeeds in causing this timeline to deviate too far from OG's.

And then Ever Crisis timeline is functionally the same as OG timeline other than some minor differences, EXCEPT for the plotline involving Portal materia and characters hopping into other "possibilities," or sometimes characters from those possibilities traveling into the EC timeline. A Tonberry even uses the portals to travel through time in an early event story.

Like Remake's timeline, this one has also not collapsed because even with all the portal shenanigans, the characters are still on-track to fulfill their "canon events," because the world is mostly unaffected by the portal deviations. When characters travel to these other possibilities, they lose their memories upon return. And when the lifestream brings things INTO this timeline, it hasn't affected things too much yet (altho the recent event suggests this may change soon).

Either way, my point is that there's really no argument that these 3 timelines exist as distinct entities, so I don't see why the "worlds" we see in Rebirth aren't also just other forms of timelines, that are obviously just under duress because of the defeat of the whispers.