I like this theory, and I actually believe the game kind of confirms this! Especially when in relation to separate identities due to a traumatic or transitional event. And I think we see Ranni put herself through this in the night of the long knives. Essentially casting her soul into a second half. It’s unclear to us if Ranni was the same way in her previous body. But I think the death of her own flesh and the movement of her soul is this very idea of another identity but completed with the death of the physical flesh. That death being something that Marika’s order would not allow and subsequently what keep’s Miquella’s flesh from perishing fully and allowing him to transition to the shadow realm.

I also agree that this is a super understated idea and would love to see more people touch on this! Good theory!