Things change in both CS I and III (remember that the prologues are a bit different from the actual times you visit those sections later on) explicitly, there are probably more implicit changes along the way due to KeA's meddling. I guess I'm not explaining things well? lol
But remember that keeping the SSS alive is KeA's real goal here, Crossbell was always going to be conquered by Erebonia, but what changes the SSS's fate (and later Rean and co's) is that KeA manipulates events slightly in order to keep them alive by bringing them unexpected allies or looping back in people who've connected with the SSS or C7 who wouldn't have otherwise. Betting on the power of bonds between people is basically the ultimate Trails trope, and it's KeA's one sole play to make.
Where that ties in with multiverse theory is that the root (a given choice/event/etc) spawns multiple branches depending on how that plays out. As long as all options are causally valid, each resulting branch is a valid existing universe. KeA natively has the ability to read that structure, but when her nature as a septterrion is activated she also gains the abillity to alter that structure. The SSS in the original timeline (or set of timelines) never get involved with Renne for whatever reason, so when she leaves Crossbell the Brights chase after her and that leaves the SSS without any backup when taking on the cult and so they die. KeA changes that by luring them into contact with Renne, her power creating new causally valid branches.
But that also means that some version of her also had to live through those original timelines (likely ending in an unaltered Great Twilight where Rean falls victim to Ishmelga as the Great Sacrifice and ushers in the end of the world) and look for a way to avert that end, and so the question becomes what happens to those other timelines then? Time is an element in this series, and we haven't seen its treasure yet. The Great Twilight seems like it would've been a definitive end of the world scenario, so what happens when the world breaks that effective causal loop (and I'm not really hinting at big hajimari spoilers here, just a general sense of 'where does the world go next')?
Also, remember that Ouroboros leadership seemed oddly sanguine about the whole Phantasmal Blaze/Great Twilight plan, and their goals don't necessarily seem to be obtaining the sept-terrions, so what is their actual goal?
But remember that keeping the SSS alive is KeA's real goal here, Crossbell was always going to be conquered by Erebonia, but what changes the SSS's fate (and later Rean and co's) is that KeA manipulates events slightly in order to keep them alive by bringing them unexpected allies or looping back in people who've connected with the SSS or C7 who wouldn't have otherwise. Betting on the power of bonds between people is basically the ultimate Trails trope, and it's KeA's one sole play to make.
Where that ties in with multiverse theory is that the root (a given choice/event/etc) spawns multiple branches depending on how that plays out. As long as all options are causally valid, each resulting branch is a valid existing universe. KeA natively has the ability to read that structure, but when her nature as a septterrion is activated she also gains the abillity to alter that structure. The SSS in the original timeline (or set of timelines) never get involved with Renne for whatever reason, so when she leaves Crossbell the Brights chase after her and that leaves the SSS without any backup when taking on the cult and so they die. KeA changes that by luring them into contact with Renne, her power creating new causally valid branches.
But that also means that some version of her also had to live through those original timelines (likely ending in an unaltered Great Twilight where Rean falls victim to Ishmelga as the Great Sacrifice and ushers in the end of the world) and look for a way to avert that end, and so the question becomes what happens to those other timelines then? Time is an element in this series, and we haven't seen its treasure yet. The Great Twilight seems like it would've been a definitive end of the world scenario, so what happens when the world breaks that effective causal loop (and I'm not really hinting at big hajimari spoilers here, just a general sense of 'where does the world go next')?
Also, remember that Ouroboros leadership seemed oddly sanguine about the whole Phantasmal Blaze/Great Twilight plan, and their goals don't necessarily seem to be obtaining the sept-terrions, so what is their actual goal?