When dead people come back, some of them as we found out, are actually impersonations of Jacob's enemy. But Jacob said to Jumbo that it's not a curse but a blessing to have the ability to see ghosts confirming that Jumbo is indeed seeing ghosts. Christian is a big mystery -- there could be more that one of him lurking around. On the Island, he told John to sacrifice his life -- that's an apparent characteristic of Jacob's enemy. Off the Island, he showed up to Jack in the hospital after Jumbo told Jack that Charlie Pace's ghost told him that Jack would have a visitor -- he's more likely a ghost in that case. But why was his coffin empty? If he's the same form as the fake John, he should have a dead body too like John.
I don't recall if LeFleur and Roger ever met in the '70s. In the future, Sawyer made fun of Roger's skeleton by reassembling him, seating him by the van, giving him a beer and calling him "Skeletor". Their meeting in the '70s could have been amusing.