Seeing Celestia show up made me headcanon that she didn't banish Luna to the moon, but rather moved her there where the taint left behind by Discord couldn't infect her. She put her back in her childhood as a sort of stasis, on the moon, knowing she'd be well looked-after by the locals.
that's a neat thought! for Moonstuck though, it's explicitly an AU about Discord's first defeat during Luna's childhood. the way it turned out, I think Nightmare Moon either happened very differently or didn't at all.