I thought this episode would have made things clearer as we approach the end but I still have no idea what's going to happen! Wonder if the light will ever be explained in that quasi-scientific manner that they did with the electromagnetism causing the island jumping in time. Also I still don't know, what
is the smoke monster? Can it be explained? If Jacob's brother died then the person trying to escape now is..his brother? Or is the smoke monster a different entity entirely who also wants to escape? Me confused.
Surely the only way MIB/Smokey can be free is for someone else to become the replacement (probably Jack), then Jack could kill Smokey/MIB/Fake Locke/etc and only in death will he be free. Then Jack has to somehow sink the island (remember we saw it under water ages ago).
I don't think the island ever sinks in the current timeline though. Bare with me, I'm trying to remember this stuff myself, but the only reason the island is still afloat is because the nuclear bomb didn't go off in 1977. In the original timeline, the Dharma initiative was building the Swan (?) site and managed to tap into the pocket of electromagnetism which very nearly caused a lot of deaths. They managed to contain it but had to build the machine which needed the buttons to be pressed by Desmond, and when Desmond decides to say 'ah fuck it' that one time, it caused the plane to crash. But last season when the island was jumping about, they went back, got the nuclear bomb, dropped it down the hatch, and Juliet managed to detonate it which caused this alternate timeline because it caused the island to sink and thus the stations never built and the plane doesn't crash.
They have it set up now that Jack wants to stay and protect the island, I don't think what he wants is for the island to sink. I don't see how he's going to anyway.