The notion of them burying MM in the church and the priest knowing just doesn't work. It's like something from an Agatha Christie plot just not realistic. I think him saying "he was deceived" is because the police convinced him the McCanns did it and he felt deceived he'd been comforting them. I don't think that they killed their daughter but even if they did they wouldn't have confessed it and they wouldn't have buried her body there.