So here is the set-up for Snowing’s big dark secret. The Queens of Darkness having been brought together by Rumple are now intent on getting their Happy Ending (and probably screwing Rumple over if possible). We know this because they tried to get the gauntlet from him in "Heroes and Villains" for that very reason.
Meanwhile Snowing are in an emotionally vulnerable place. They had taken back their kingdom only to have their fairytale wedding ruined by Regina declaring she would destroy their happiness. This really gets to Snow as we see in 3.10 where she admits that she wants to have a baby but can’t move forward with Regina’s threat hanging over them. At the end of that episode she seemed ready to move forward despite her fears but given the sneak peek it is clear that she is still troubled when they return from their honeymoon.
The QoD come to them with a proposition. (Maleficent magnanimously forgives Charming for attacking her…does she even know she has a True Love egg inside of her dragon form?). I don’t know the specifics but I am hoping it deals with Maleficent wanting to get her child back. My guess is that King Stefan took her baby which is what led to her cursing Aurora as payback. Getting her child back would be her Happy Ending. In return for their help they promise protection from Regina. Seems like a solid deal so the Charmings agree.
While trying to keep their promise they get a better offer from The Dark One. Depending on how dark Adam and Eddy want to play this Snowing either consciously decide or are tricked by Rumple into betraying the Queen’s. Either way the baby they are supposed to retrieve ends up in the LWM as an orphan; lets call her Lily. Yes the same Lily teenage Emma met and felt betrayed by; and now regrets walking away from.
The Charmings of course feel horribly guilty for what they did. Which explains why they went above and beyond to help Ella/Ashley when Rumple threatened to steal her baby. It also explains why Maleficent was interested in the Dark Curse enough to trade her sleeping curse to Regina for it. Of course, like Rumple, she couldn’t enact it because the thing she loved most was in the Land Without Magic. This means that she is using reverse psychology on Regina in 1.02 in order to get her to think she was stealing the curse and give her incentive to bring Maleficent along to the LWM. Of course Maleficent probably didn’t plan to be a dragon for the whole thing or get “killed” by Emma. This of course is exactly what Rumple planned since he needed the magical dragon version of Maleficent to come over with the True Love potion.
Phew! Did you get all that? Okay good.
Now the genius of this is that for Emma it is a two pronged assault on her relationship with her parents. Because she understands what it meant for Lily to grow up an orphan and because she understands how it must have hurt Maleficent to lose her child (hat tip to always-comment-on-the-mutton for the mother angle idea). I think discovering her parent’s secret would be enough for her to decide to actively help the QoD (and perhaps that is turning Dark?)
*Another hat tip to thisisevenharderthannamingablog for her excellent timeline that I used to make sure I had events straight!