Here we go! We are almost to the end! But here are a few more theories to throw out there. So I believe that the Man in Black was trapped in the cabin at one point but the ash was broken before 815 crashed and he has been free to roam around and he hangs out in the cabin on his down time.
Now when 815 crashed he found Christian's coffin and took his body out of it and hid it in the cabin. This is why the coffin was empty. We know that he is pretending to be John Locke but Locke's body is still around. So here's what I think. He hid Christian's body in the cabin and used it when he wanted.
So later when Hurley looks in the cabin he sees Christian sitting there and then an eye, I think it was the Man in Black's eye and just Christian's body sitting in the chair (He was in a suit if I remember correctly but don't remember if he was moving around in the chair.)
Or maybe the Man in Black pulled a stunt like for Richard when he changed real fast from Isabella to the Monster, and changed from Christian real fast to himself and popped up in the window.
So then later, perhaps Christian's body was hidden in the floor boards or something and got burnt up by Illana and co. What do you guys think? Any holes in my theory?
See you in another reality Brotha!