The only problem I have with this theory yall throwin around, is why would Desmond be all calm around Penny? He was acting as though he had a vauge idea of who she was, but you would think that if he went thru as much shit in the original timeline for her as he has & the fact that they have a child together, he would be acting a lil more frantic, trying (and failing) to get her to remember their life together. Not calmly trying to get a date with her. Also when Desmond woke up in the room, he wasn't confused at all by the people around him. Also when Charles went to explain his plan, Desmond cut him off saying that he understood and it was no problem, why would alt. Desmond do that if he would have no idea what is going on. Maybe Desmond has an idea of what is happening and that's why he went with Sayid.
Maybe what happened with everyone is this analogy; the original and alt. versions of the lost crew live in the same room but looking thru windows on the opposite side of the room. Everyone has a wall in the middle of the room so that they are not aware of each other's existence. With Desmond, Charlie, and Faraday (altho he's Widmore in this version) (Maybe Eloise and Charles Widmore, I'm not sure), the wall is crumbling in the middle to the point where they can catch a glimpse of each other. Only difference between Desmond, Faraday, and Charlie is that only Desmond is alive on this side.