Well think about it this way, they have the memories of both lifetimes so Snow has spent just as much time being MM as she had being Snow. Charming on the other hand was asleep for all but a few months so he’s a lot less of David then Charming. Charming will always think of his wife as Snow, but Snow is just as comfortable thinking in Storybrook terms then FTL. Charming referred to her as MM because he was talking to Henry, who only really knew her as MM outside of the pages of his book. (if any of that made any sense).
That’s why it’s all gonna be tricky with them going back and forth on the names not saying I don’t like it but I just think it’s gonna be funny