So I'm finally able to talk in this thread after having finished the game.
- I believe that everything with the multiple worlds is in the lifestream, in a "dream" basically. The multiple worlds are the what-if dreams made up of the 'memories' of the planet.
- Aerith is dead. Sephiroth killed her. BUUUUT her consciousness is still alive in the lifestream. That's how she's talking to Cloud. Cloud is clearly still hallucinating at the end of the game; his mind is in the in-between world where Zack, Biggs, etc. are "alive" even if physically he's still with the rest of the party. That's why when the perspective cuts from him to the rest of the party, Aerith is no longer there.
- Zack, Biggs, etc aren't actually alive, but their consciousnesses are preserved in the lifestream for some unknown reason yet.
- They went for maximum confusion in the ending with the white whispers representing Aerith and the black whispers representing Sephiroth and multiple groups of people fighting multiple Sephiroths in multiple worlds. But it boils down to this - Sephiroth wants to change the fate of the world, and so he's trying to manipulate and control the lifestream and corrupt Cloud.
- I really, really love the part in the Temple of the Ancient when Cloud chases Aerith laughing creepily. That gave me goosebumps.
-This game is legit not a sequel, it is a reimagining of FF7. However it's also a spiritual sequel in that it relies on the audience having played the original to get a lot of the subtle things they're doing in the game. I feel like the devs are messing with us the same way Jenovaroth messes with Cloud.
Also I just realized that Aerith is protecting Cloud's mind from shattering by presenting him with the illusion that she's still alive. Very interesting.