I think doxcen and hjbau both have excellent theories!
My one issue with the recipe for the dark curse is where this falls in the timeline. We don’t know how much time is between Snow and James first meeting and the curse. We do know that the Magic Mirror knows that Evil Queen traded the dark curse wi Maleficent for the sleeping curse. This would mean a fast passing of events for Rumplestiltskin to make the dark curse, give it to EQ (or it be stolen?), EQ to trade with Maleficent, and Maleficent have some personal experience with it, and EQ to get it back.
It could all happen within the time we have left, but that would be tight. I doubt we’ll see Sleeping Beauty this season, so we may not see Maleficent, either.