But wouldn't that ^^ negate the idea that Cora was the miller's daughter?
Yeah, "The Miller's Daughter" pretty much implies that Cora is going to turn out to be part of the classic/original Rumpel story where she promises her firstborn child in exchange for marrying the king.
It's a good thing everyone is limited to being in only one story!
(Note, also, that no one mentions which
king. And assuming she did give up her child, she'd probably want another one pretty badly, be it by hook or by crook.)