What do you mean David and Regina end up together? Like romantically? I doubt it and i really hope not.
Maybe Mary or Emma will wonder about Regina’s motivation and think about why she would be setting Mary up. That is a good idea. Get Emma to believe. I would actually rather Snow get her memories back this season then Charming, but there has been a lot of references to Charming and he had a flashback in the last episode so they are leaning more towards it being him.