You can very naturally slot in Cloud/the party remembering Aerith's death to the return to FC. Cloud is already reminiscing about Aerith's death there in the OG, this would merely be an expansion. I'm sure the clear White Materia will be involved somehow too.
Cloud's true self/his memories will have already been restored post-Lifestream. He will no longer be "delusional," per se, he'll have accepted her death, but he still has to confront it when they return to the FC. But again, this is nothing new, this is just following the trajectory of the OG. Anyone who thinks this somehow diminishes Cloud's feelings for Tifa is never going to be convinced anyways, so who fucking cares? lol
This is pretty much how the OG treats Cloud remembering Zack post-Lifestream. Cloud remembers him now, but the memory of his death doesn't come to the fore until they return to Nibelheim.
I just don't understand what Cloud remembering Aerith's death during the Lifestream sequence would solve? Cloud was already delusional before she died (he was delusional before he even met her!), so it'll just restore him to a slightly less delusional state while doing nothing to address the root cause of his mental break? That's a narrative blind alley/waste of time that would actually diminish the impact of her death.
Adding her to the Lifestream sequence makes as much sense as adding her to the Wutai subplot, in that it's certainly something they could do, but why would they? How would this improve upon the OG in any way?
That's what I'm asking when I see all these weird-ass predictions/dooming about the Lifestream and Highwind scenes. They aren't going to make things different just for the sake of being different. Remake/Rebirth has added a lot of new lore to the story, but when it comes to the character-forward moments like Barret/Dyne and Red/Seto, they've been pretty damn faithful.
The way they've handled Aerith's death in Rebirth feels like a pretty significant change, but it also tracks with it being something they had intended all along but lacked the technology to depict in the OG. Meanwhile...if they wanted to have other people in the Lifestream, or if they wanted you to have different partner options for Under the Highwind (lmfao), they obviously could have done so in the OG.