Lock has always been dead by Lostina

The MIB has always been in control on the island. He was the one that got John to leave ( posed as Christan) knowing that he would die, to bring the others back with John's dead body, so he could take it over and be reconized as John still be a leader, and get Ben to kill Jacob. It was his plan all along. After all how do you get people to follow you if your Christan Sheppard when no one knows who you are. He needed John's body so he could be in charge. Remember Christan already tried to get John to turn the wheel which resulted in Ben doing it instead and creating the time shift. So he tried again getting John to turn the wheel and it worked!

This is the loophole that ends up getting Jacob killed.

However, when they come back, the actual candidates are in a different timeline. Where, Ben Flock, Sun are all in the real timeline. Being that the candidates Kate, Hurley, Jack, Sawyer, Jin, are back in time allows the loophole to killing Jacob. I don't think if they were all in the same timeline that Jacob would have been killed. They were the Pawns, if you will. They protected him regardless of knowing that they were.

Jacob goes to each one of them as kids, and adults to touch them. This is how they get chosen to come to the island. His invite. However it is'nt working if they are not all together, and not on the same time. SO... He finds Ilana and invites her to come protect him and the remaining candidates. She comes to the island to help Jacob, and is on the same time line, except she was not able to save him as well.

The island is basically done with you, when you can no longer serve your purpose to protect Jacob. This is why eventually Ilana dies. However the other are still Candidates in running for Jacob so they remain alive. Unless they become weak, and therefore cannot serve there purpose as a protector such as Claire and Sayid. They then can die.

If john never left, the MIB would not have had his loophole. Although they believe to stop the island from shifting through time they needed him to leave, that was not the case. It was just a ploy to get the loophole to work, and it did.
However they would still be time traveling with those annoying sounds and nose bleeds if he did'nt so I suppose it had to be done regardless. If Ben never turned the wheel though that never would have happened, cause he was not suppose to turn it in the first place as I already mentioned. Either way if John originally did, he still would have died off the Island. They still would have come back with his body for the MIB to take over. Except the Oceanic 6 would have trusted Locke (MIB) knowing that he was dead and that he was special which would have still eventually lead to Jacob being killed by Ben. They would have all been on Lockes (MIB) side. Really it worked out better for him that the time shift happened. It resulted in it being easier for him without them all there.

Looking forward to the comments guys.