I'm still going to maintain the belief that Remake is actually the memories of Cloud and co., who are now dead (simply because of old age) entering the Lifestream and pouring their memories in it; Remake = Memories of the OG that the characters have. The OG has become the Planet’s memory with their passing.
That explains the small differences from the OG. Our minds embellish and forget. We never remember the things exactly as they happened. The Remake having some minor differences is the characters remembering the OG actions slightly wrongly. Take the flower, for example. In the OG, you have the option not to take it; here you take it no matter what. You had the option who to give it to; here Cloud gives it to Tifa and it suddenly means Lovers’ Reunion. The OG is the real deal; the Remake is how one character (possibly Cloud himself?) remembers it. That doesn’t harm the Lifestream flow; it’s natural we forget details of the past as we go. It’s the big events, the skeleton, the outline that remains.
This helps with the Remake and the OG co-existing, not deleting each other and allowing SE to get money from both; they’re not replacing anything, that’d be financial stupidity and harm.
The bigger differences exist because Sephiroth is messing with the Lifestream itself, since he continues to linger in it. Now that the characters’ memories have entered the Lifestream and “no one’s dead if we remember them” well, of course when entering the land of the dead, the Lifestream, Sephiroth will be powerful. He’s remembered very strongly by the characters and the Meteor he summoned is remembered by all. It burns eternal in our hearts. Not just as himself, but the memory of him is now in the Lifestream, adding to his power. Especially from Cloud’s memories. Adding the JENOVA cells that cannot be dissolved since they’re the alien element and you get Sephiroth in the Remake.
With all this power coming to him, Sephiroth is able to fulfill his words in ACC: he will never be just a memory. He’ll mess up with the memories of the OG characters. Messing up with the characters means messing up with the Lifestream and its natural flow. And messing up with the natural flow of the Lifestream… well, it kills the Planet. Sephiroth wants to enter the cycle while also keeping his personality intact and he has found his “home” in Cloud and co.’s memories of him. He messes up and touches everything while also being untouched by them, looking for the beginning of creation, the source of the Lifestream, in order to harm it. Harming it will kill the Planet and allow him and JENOVA to use it to travel into the cosmos to invade other planets.
But, Aerith exists as long Sephiroth exists. They’re Yin and Yang, Black and White. She’s a native of the Planet, so she follows the cycle and she has an… expiration date, let’s call it. She’s weakening, but still existing. She realized what Sephiroth’s plan is and sent her friends’ remnants of spirits to try and put things back as they were, fix the BIG alteration of the memories. They’re the whispers, the people from after the events of the OG, maybe even the versions of the OG characters who went on to live. They fight to remain versions of themselves that happened after the OG. If the memory of the Planet about them changes too much, they will die. Not just dissolve into the Lifestream; they’ll disappear. They’re Aerith’s allies, the ones who understand what her death and all the OG events lead up to; a living Planet, a happy ending, a life going on.
Sephiroth hates them, but he has realized that he can use them. He fools Remake!Cloud to get rid of them in order to alter their “fate”. He’s not lying; he’s telling Cloud the truth. He can alter the memories too much by getting rid of the Whispers, the ones who correct the bigger changes. He just calls it “destiny” instead of “memory”. But Remake!Cloud is a memory himself, he’s Cloud as he remembers himself in the OG and he’s completely unaware of it. All he sees is the people he cares for dying, being in danger… he’s unaware that they HAVE to die in order for the cycle to continue, for life to go on. He’s unaware of his happy ending. All he sees around him is death and suffering (the OG was NOT a happy little story, wasn’t it?) and those whispers sometimes adding to it. So, that young, unwise, trigger-happy Cloud attacks what Aerith sent to help and gets rid of them. Did he kill a version of himself? Did he kill ACC Tifa in that final battle? Probably.
And then, Sephiroth gets powerful. So powerful that he steals Aerith’s Whispers and uses them, in order for Remake!Cloud to get rid of them. And he’s successful. “The future is not yet written.” Finally, he’s free of the Whispers and he can alter the memories as much as he wants. Alter them enough for everyone to want Aerith to live. After all, nobody in the Remake world wants Aerith dead, right? That’s a bad thing that happens. They’re unaware that her death leads to the Lifestream defeating Meteor and allowing life to continue.
That’s when dream!Aerith enters. She comes to Remake!Cloud and talks to him and all her words are of the one who knows the truth: you made me happy, don’t fall in love with me; it’s fake. It’s essentially “Thanks for remembering me fondly, but I have to die. Don’t get too attached to this version of myself, because if you change my death in your memories and start truly believing that I lived, things will get distorted too much and the distortion will spread.” Remake!Aerith says that whenever the Whispers touch her, she loses a part of herself. It’s because she’s fading herself; she’s a Cetra, but she IS of the Planet and the cycle caught her, too. She cannot be ACC Aerith; knowledgeable and wise. She’s far too gone for that. And she has to play her part and not alter the memories too much herself, otherwise the same harm will come. So, we got Remake!Aerith. A mixture of ACC Aerith and OG Aerith, or at least, as Cloud and co. remember OG Aerith.
Yes, there are a LOT of faults in this theory. But I’m sticking to it, because I haaaaate time travel and paradoxes and different realities.
At least this way, there’s no sudden new element that we’ve never heard of before entering the lore; aka time travel. There’s only the Lifestream and memories; something that were already there from the OG. This way, we also have the final battle against Sephiroth. Fucker continues to mess up with the Planet for only after his death, but after everybody else’s death. By defeating him and JENOVA in death, that’s it. The story ends. We can have Nanaki and his cubs 500 years later, without any concerns, as the absolute finale of the Compilation.