This whole thing with Jack is related to Desmond's supposed ability to see into the future, when in reality he is reliving the past. I think they are stuck in some kind of time warp or something like that.
Locke is working for the island, he's taken over for Ben in that regard, but why did the Island dump Ben for Locke? The Island has always been kind to Locke if you will. He can walk on the Island, the black smoke defense mechanism left him alone, mostly.
How about Hurley!!! He came bustin all bad ass into the camp and took out those guys with the van. That was freakin' great!!!
As for who is in the coffin, I think it is Locke. Don't have a reason why yet, just my gut reaction.