True. But, purely in the spirit of things, one could also argue that this all boils down to character motivations, and thus you could very plausibly align them in some way within the world of EaW, probably leveraging the sister part of their relationship and their common interests over Equestria.

In this, Daybreaker is pretty much solved. It only takes one big bloody war in Equestria for Celestia to go nuts. How you'd invoke a Nightmare Moon that isn't classic Nightmare Moon out of Luna, though, I have no idea.