Short theory, but I realized something huge that I overlooked. If Richard has always known Jacob was in the statue, and it took two people to move the huge block to get in there, how would Richard have been the only one to see Jacob? He would have always needed some help to move the damn thing. I'm guessing Ben has met Jacob several times. And he knows exactly what is going on. Like the other theory about Ben lying and that is what he does, Ben "Always has a plan". Ben is the master manipulator, and he has his own agenda, which I believe is Jacob's agenda. Ben has always know the entity in the cabin was the man in black. Due to all the time travel going on, he has always known exactly how things are supposed to turn out up to a certain point. He is simply...Ben!
