I gotta say, that was not what I was expecting of that backstory. At what point does Cora learn more magic? Doesn’t she even say to him in Storybrooke, ‘No, I don’t know any memory spells, I only know what you taught me.’ Now that’s probably not entirely true, but I expected it to be at least remotely true!
Maybe that was her sarcastic way of saying, “Dude are you serious, you have no idea how powerful I am.”
There had to have been at least a short passage of time between the proposal and the wedding day. Those royal shindigs don’t just organise themselves overnight. I suspect he was visiting her every night for a little while, and teaching her more things, and it’s during that time that he gave her the spell book to study.