I think the heart is David’s because of how strange he was acting. But, because of how strange he was acting in the woods vs. how he was acting normally in SB, I think Regina removed his heart very VERY recently. As in, some point during this episode that we never saw. As far as MM’s fingerprints being on the box…is it possible that Regina has some sort of magical control over MM and got her to do the deed of ripping out David’s heart without her MM knowing/remembering? Thus, Regina could have MM take the fall for Katheryn’s disappearance AND have David under her control thus ensuring that their happily ever after really can’t come true. I’m sure there are a lot of holes in that theory but I think it’s gotta come down to Regina somehow.