There's no way to give him complete control and have him not know who he's controlling. Why does he need that restriction anyway?I'm starting to think that David shouldn't have control over the posts and votes because that way he could learn the identity of the player.
I think you can go two ways with it. You can give him complete control, provided that the controlled player has some way to fight it and potentially break it (such as the shrunken head example). Or you scale it back to night abilities only, in which case ToS's witch does an excellent job as a mechanic that's not complicated and easy to specify.