Sorry if this is kind of disjointed! I've been thinking a lot lately about Rei's symbolic role as the priestess in FF3 and what that means. I thought it was interesting that both Miku and Kei start dreaming about the Manor quite early on, but it's not until Rei finds out about it from them that she begins sharing their dreams. If she's just picking up on people she knows are suffering from the curse already, why doesn't she also share Mio's dream, or Yoshino's?
The theory/headcanon I came up with is that once someone's crossed a kind of event horizon of trauma and grief - once they've lost all will to return to the real world and chosen to follow the dead into the Rift - Rei can't share their dreams any more. In Miku's last cutscene, when she decides to follow Mafuyu, she and Rei are forced apart, and Rei's voice can't reach her any more. I think the same is true of Mio and Yoshino, that the curse is so advanced with them that they're basically ghosts as far as Rei's concerned.
So far so good, I was quite pleased to have a little theory about that, but it doesn't really go anywhere. Then I was reading one of the columns Makoto Shibata wrote for the official FF3 website (I recommend you read it too, it's really interesting!), and there's a really quote in there about the ending that made something click for me:
Regarding the protagonist, Rei Kurosawa, though the pain of living on never goes away in the end, she realises that having lost Yuu is the reason she must go on living, and though she wants to be with him, Yuu's message is that the death of Rei would effectively kill the living and dead people inside her, and though it isn't said outright I wanted to give that to the player.
The fact that he said "the living and the dead" just made a !!! go off in my head. He might just have been talking metaphysically, but my immediate thought was that he was talking about Kei and Miku. By that point, though technically still alive, they have crossed the event horizon from which you're not supposed to come back. Rei, who remembers them and formed a bond with them through their dreams, is like their last connection with reality, and I think that if she'd chosen to go with Yuu, they would have been lost too. But since she chooses to live on, she literally keeps them alive, as well as figuratively keeping alive the memory of Yuu and all the other dead ones.
All this is assuming the second ending as canon; if this is right, it's probably not good news for Mio in the first ending, since her connection is through Kei, who's already gone. Feels a bit strange for me to be saying this, since I've always been in the "Mio survives in both endings" camp.
So... yeah. That's my theory. Does that make sense to anyone except me?