Snow and David didn’t know about the dagger till Snow saw Cora and Regina digging and talking about the dagger. Since August is a writer he might have reseached fairy tale stories or his comment about a fairy told could be true too but it wasn’t in the book and as RG stated that Henry had a hard time figuring out who Gold was.
Yes, the BF did do the the adoption as nuns usually do handle adoptions in the old days.