NOTE: This theory has may have already been explored but I’ll hit you with it anyway.
At some point, at least from the time when Richard visited young John Locke, The Nemesis had been instructing Richard/the Others on what to do, and doing so as if he was Jacob. He sent Richard to see Locke because he wanted Locke to be special, so that he would be bought to the Island and could be used as the ‘loophole’. The Nemesis was ‘the man behind in the curtain’ who asked John Locke for “help” in the cabin; knowing that this would anger Ben, and Ben subsequently shoots Locke, which, had Locke died, would have given The Nemesis the chance to take his body. In this case, ‘Jacob’s Cabin’ was in actual fact ‘The Nemesis’s Cabin. Also, this is why Ilana went to the Cabin: to see if the fake Jacob had escaped.
Also, it is The Nemesis that writes ‘Jacob’s List’, deliberately not including Jack, knowing that Jack’s subsequent chain of events would lead him to 1977 where he influences ‘The Incident’, which as it turns out, will not favour The Nemesis’ objectives.
In actual fact, Jacob has all the time been aware of The Nemesis’ plans, and, has been subtly manipulating events in order to favour his motives. Not too long after Richard sees the not-so-special Locke, the real Jacob lets Richard in on the plan. Jacob gives Ben the tumour so that Jack ends up on ‘Jacob’s List’ thus countering The Nemesis’ intentions. Jacob’s motives behind reviving John after being pushed off by Cooper are unclear, but could range from the following:
1. He did not want Locke dead because he did not want to risk having a dead Locke for The Nemesis to even have the slightest chance of using him as a loophole.
2. He wanted Locke’s life to transpire the way it has so far in the series, because it was Jacob’s plan to let The Nemesis have him killed by Ben, but only at a particular period in time (which is why he hindered previous attempts).
3. Jacob had a greater plan for John Locke, although it is unclear whether that plan is still in effect now that both Locke and Jacob are dead. (I like this one the most, because it could mean the proper Locke coming back!!!)
There’s probably some fatal flaw in this theory but I couldn’t be bothered to check the specifics... just went off memory.