You truly are missing the point
@Obsidian Fire. What we are saying is that no one else can help Cloud in the Lifestream because Tifa's the only one he'd let come so close and know such intimate details. They are calling each other and yearning to get reunited. I think you are totally missing the whole point of the scene, and the roles the characters are given.
Let's recap, why is Tifa a heroine? Because at first, her character was one with Aerith. When they decided to create two heroines, their roles were split: and so, Aerith is in charge of the overarching story, while Tifa is in charge of the intimate plot of the story. One helps the planet, the other helps Cloud. The LS scene is, narratively, not only the resolve to Cloud's messed up head, but also the resolve of the love triangle. It is here to tell you about Cloud's deepest secret - a secret he doesn't want to share with anyone BUT the girl he's always been in love with. It literally does not matter if anyone else who knew him before the Nibelheim incident is here because that's not them he wants to see. He says it in Northern Crater "only your opinion counts" or something alike. He doesn't care about ANYTHING ELSE. And he tells the player so in Northern Crater.
Let's also note that Remake already hints at the Lifestream scene with scenes from when he was a kid. Those, Zack wouldn't know how to resolve them. There's a reason why the real Cloud is represented by himself as a kid: it's because the root of all of this is Tifa falling in the mountains and Cloud failing to save her, then getting blamed. Again, this is a story that only concerns Cloud and Tifa. But in Remake, we see Cloud making up memories about them as a kid. In chapter 1, he makes up a scene where Tifa notices him when she didn't. They are underlining this in the next Ultimania, and if it wasn't important, they wouldn't. But they are saying "hey look, he's making up a scene where Tifa notices him!" Why? We know why. Because he wanted to be noticed. And from chapter 1 that's being hinted at. You can't have Zack there. He doesn't have a place.
Ultimately, you have to also look at how Nojima wrote his pairs for FFVII & Compilation: he entwined their stories. Retroactively, he entwined Zack and Aerith as much as he could, and he did it from the start for Cloud and Tifa (look, they even have matched outfits in Remake for the dresses with matched accessories). The Lifestream scene is that: Cloud and Tifa share the same, deep story. People have complained for years that Tifa didn't have a backstory, but that's not true; she has nearly the same as Cloud's. It's their story unfolding in the Lifestream scene, you cannot have someone else replace Tifa.
