With all the talk of Nikki and Palo surfacing I just wanted to toss something out for consideration. When the mystery man turned up disguised as Locke, there was a body still in it's casket which Ilana and company transferred to a crate so they could take him to Richard.
Christian on the other hand has never really been proven to be dead on the island. No body. Though I believe Christian has been imitated by both Mystery Man and Jacob, I also believe he is very much alive. I believe this is the only significance Nikki and Palo can lend to the Lost story, because of the effects of the Medusa Spider.
Widmore owns a large and lucrative pharmaceutical company. He easily could have taken the venom of the spider and had a serum created to make Christian appear to be dead, even to a doctor. The coroner could have been payed to make up a story of what caused his death, and off to the island we go.
I believe Christian is the one who disturbed the ash prison and set Mystery Man free. I also believe that Christian convinced Claire to abandon Aaron, assuring her that Sawyer would find him and get him to Sun. I don't think they imagined that Kate would be the one to care for him.
Ben and Richard both have said, even on the island, dead is dead. But I believe Christian was never dead to begin with.