I completely agree that the hitch in the theory is that Regina’s father does not seem like a king to me. I think that since it seems like whatever happened between Snow and Regina has something to do with the stable boy and the princess liking each other so Regina is a princess and her mother is a queen. Maybe like you said Regina’s mother had a child with Henry and he wasn’t the king or maybe the character is just different though i am not sure that quite makes sense because when the Queen killed Snow’s father, her father was working in the castle there as well. I think we just need a bit more information. It is a good theory though even if the story doesn’t quite follow the original Rumpel story it could be what the original story is based on.