We've always known Jenova has some mimicking abilities, but an ultimanium article goes on to say that anyone with Jenova's cells also has some degree of mimic. It's said that not only can Jenova mimic the look of someone, but it can access their memories and mimic that. They go on to claim that when Cloud is first found by Tifa at the train station, he's not only not himself, but the Jenova cells are crafting a personality of Zack + Tifa's memories of Cloud. They go on to give examples of those with Jenova cells showing other mimicking, such as Zack mimicking moves from others, and Cloud mimicking Meteor Rain.
It's always known memories are a big deal in FFVII, materia being the biggest example. He uses multiple examples, from the DMW in Crisis Core, to memories of those Cloud was protecting allowing him to overcome Sephiroth in Advent Children.
They go on to claim with a pretty convincing argument that what we're seeing in Remake isn't time travel by Sephiroth or Aerith, but memories from the future being shared back, due to what Sephiroth is doing. This is being done because Sephiroth is taking control. All through OG FFVII, Advent Children, and some parts of Remake, the Sephiroth seen is either fully controlled by Jenova, or being actively influenced by her. He's just as trapped as anyone else, just a pawn in her schemes. Her endgame is the destruction of the planet, and while the crew prevent it during Meteor and Advent Children, eventually, the planet is destroyed.
Instead, Sephiroth wants to go back to what he originally said after learning of the ancients, that the planet is his birthright. He wants it for himself. Due to seeing the future memories, he knows that Jenova will get her way, and that Cloud and co. are capable of stopping him. Due to some ultimaniums and such, the guy found excerpts that specify that when Sephiroth entered the lifestream, he discarded memories that were no longer of use to him, such as his childhood, training in soldier, friends he made, etc. Those all were absorbed into the lifestream as any other memory would be.
He discusses how Aerith, Zack, and Sephiroth are all part of Cloud, living on within him. Zack and Aerith's roles are to ensure he wins, effectively, while Sephiroth just hides within his memories.
I forget exactly the details he gave on why he believes this, but based on what Sephiroth is saying in Remake, he's saying that Sephiroth plans to come back based on memories. But since he discarded those to the lifestream, the only memories remaining are those that Cloud harbors, thus, in Remake he keeps approaching Cloud from within his own consciousness, stoking those memories, making them a focus, so he can strengthen and come back. This is sorta shown, as no one besides Cloud can see Sephiroth for a long time, then finally in Hojo's lab, after Tifa touch's Cloud, the whole party sees him.
Again due to some ultimaniums and interviews, it's mentioned that Jenova was known as Destiny Core, the start of destiny. Prior to her entering the lifestream with Sephiroth, there wasn't destiny. Once entering it though, and corrupting it, gathering memories from it, she was able to alter the course of the world to her liking, resulting in the ending in which the planet is destroyed.
With Jenova infecting the lifestream, she was able to also take over the whispers and arbiter of fate. This is why they attempt to not only stop the party from changing course, but also Sephiroth, like in Shinra Tower or on the highway. She wants to preserve the course of events where the planet is destroyed in the end.
Sephiroth antagonizes the party on the highway for the purpose of inciting the whispers to action. It's why they all gather there to stop him. With the party pushing through and destroying them though, it leaves Sephiroth the opportunity to absorb it all. Since the memories they contained would include his own, that furthers along his plan of coming back, while also absorbing Jenova's consciousness.
Aerith knows she will die, and that it'll negatively effect Cloud for the rest of his life (at least to Advent Children). She doesn't seem opposed to that outcome, but she knows that they have to stop Sephiroth's plan because it's unnatural and against the order of things.
Knowing that Cloud is capable of stopping him, part of his goals also appear to be to separate memories from him of those who could potentially help him, such as Zack. It's why in the ending, in the white memory space, Zack walks past him without him noticing. I dunno how that'll work exactly since Cloud can't be Cloud without Zack, but that's what he's going with. It's also why at the edge of creation, Sephiroth gave Cloud the chance to join him, as a merciful gesture. Otherwise, he'd just fuck with him til he's broken him.
Part of seperating Zack from Cloud and Aerith has left him alone on his own path, with no support from them, no resolution or goal of his own story. It's why when he opened the church doors and had the white background, which usually indicates something going on within memories, there's no one there. What'll come from that, who knows.