So, the only reason that Locke thought he had to die, and told Ben that he had to die, was because Impostor Locke told Richard to tell Locke that he had to die, so that he could come back as his replacement. But the impostor knew everything about Locke and the exact time and place to meet the wounded, time-traveling Locke, and knew that the door was from the hatch where Locke and Ben first met. So the impostor is not only ageless, but also omniscient like Jacob. And since Jacob is all-knowing, and recognized the impostor right away, he had to know that Ben was going to kill him and let it happen anyway, kind of like Obi-wan. In fact he egged Ben on with his response to Ben about why he never got to see him.
Jacob knew where to find all of the future island inhabitants right at a moment of great conflict and help them out a little, maybe thereby selecting them as the future inhabitants, except he didn't see Hurley and Sayid until after they had come back from the island. Did he also go see the other future crash survivors like Charlie? He also went to that Bosnian or Croatian woman in the hospital and told her he needed her help, so I'm guessing he knew that his old buddy was coming back to the island.
What's the loophole? I'm guessing it is that only a leader or former leader could kill Jacob. So by pretending to be Locke, he was now the leader or maybe he just needed Ben since he was a deposed leader. And if the impostor is the same guy from the 1700's or 1800's at the beginning of the show, shouldn't he have known that Jacob lived at the statue?
When Ben was in the temple and the island took the form of Alex and told him not to kill Locke and to do what he said, the island had to have known that it wasn't the real Locke, so maybe the island was for getting rid of Jacob as well.
In the training videos, the scientist guy had a fake arm in some of the videos, so now we know how he lost his arm. But that would mean that despite blowing up the nuke, everything goes on as before, I guess. Also, if originally they had hit the pocket and caused the problem, how were they able to ever build the Swan on top of it?