I think you could be pretty much on the right track with a lot of what you wrote.

Although I now agree that it was probably never Jacob in the cabin but a trick by MIB, I wonder why there was the ash ring around it.
That would be like Superman posing as his enemy but putting Kryptonite around himself.

Since you mentioned a lot of people showing up: How about Anthony Cooper, Locke's dad? The whole magic box talk never lead anywhere. Could it just have been another test by MIB? Or Jacob?
Walt is still the one I wonder about the most. After what MIB told Jack this time, he really can only take over dead people after all.
It was Walt who "helped" Locke out of the hole when he was shot in the missing kidney. Which is another thing why I don't want Locke's existence to have been in vain: That hole missing kidney saving his life made him out to be too important not to survive! That couldn't all just have been MIB's plan now, could it?

The whole testing of MIB and Jacob with the candidates and other people fits to what (I think it was) Cannoli wrote a few weeks back, that Eko was killed by MIB because he found him to be incorruptible.

I am really looking forward to re-watching the show once it's all done. Unless they really screw up the last few episodes now. There are just a few things I could not live with!