I don’t know that because Regina hasn’t killed Emma proves that she doesn’t have magic. There are still guns in the real world and she hasn’t shot her either. And i think she could get away with it. You could be right in that Regina might not be able to use magic like disapparating or throwing fire balls, but is still able to do things like use the mirrors. I don’t know.
Maybe if the curse is partially broken and becomes less powerful then they might get some of their power back.