I think he knows about the dagger because he’s been working with Cora. My theory is that Cora knows all about the dagger, that’s what the battle was between her and Rumple. While I do think she is the miller’s daughter, it can’t be his “guess my name” thing from the original tale since Rumple has a habit of introducing himself to people.
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"He was a lot of things to me" "The only conclusion was love"