Who was it that said that each side came from a different timeline? It seems to be leaning that way. Although this would be too complex of a plot and kinda hard to execute without the question "then where are the others?", like alternate universe versions of them or something.
It could also be memory manipulation since the beginning. Someone must have tempered with something that might have either caused memory fabrications from one side or made the whole ordeal's ending be stuck in some kind of Schrödinger-like limbo or smth — we don't know who but my guess is the Demon Queen did it out of grief and she really did kill the hero (hence why many of Kurose's memories seem to be "blocked", there could be an emotional reason to that).
Anyways, Rinka your gay is showing. Don't try to do do something that you'll regret out of jealousy. I think she won't, but still... sometimes we consider some terrible stuff out of rage (Kurose/Dia being the Demon Queen, the big ol' enemy, and the one who "stole her childhood sweetheart friend" is a good enough reason for both past and present versions of Rinka to hold a huge grudge).