But the "Nuke" (I still don't think it was nuclear since they only removed a small portion of the bomb and the "nuclear" part of a nuclear bomb is the big, heavy part that makes the bomb so big.) couldn't have caused the island to sink. Why? because the island is still there in one of the two timelines we are watching, but it went off in both.
The timelines, as far as we know, diverged when the bomb went off, but it still went off in both timelines. So it couldn't have directly caused the island to sink in one timeline, but not in the other.
Well, if they were going by JJ-Trek rules, then the point of departure in the timeline wouldn't be when the bomb goes off. It would be when the Losties first show up from the future. So you'd have:
prime timeline: Losties never lived in the 1970s. The bomb never goes off. The Island never sinks. Flight 815 crashes in 2004.
new timeline: Branches off in 1974 when the Losties show up from the future. They end up setting off the bomb in 1977 and the Island sinks. When the bomb goes off, they're sent back to 2007 in the prime timeline.
Of course, there is the problem of the photo which, as Lindley notes, "proves" that the stuff in 2007 is actually happening in the same timeline where the Losties were part of Dharma in the 1970s. Though as Lindley also notes, the photo looks a bit different from the scene we actually see in the 70s.
I guess the main question is, when Faraday shows up in "The Variable" and spouts off the gibberish about "Oh, we're human so we have free will, so we *can* change the future", was that the writers' way of announcing that they're changing the rules away from the previously established "whatever happened, happened" time travel rules to something else, or was that just a red herring? And if they are changing the rules to something else, then are those new rules actually logically self-consistent, or do they just seem like they make sense on the surface, but don't really make any sense if you think about them hard enough (which is how most time travel in Trek works)? If the latter, then it'll be impossible to reason out what's going on, because the underlying logic wouldn't make any sense anyway.
For myself, I'm all for them sticking with the WHH time travel rules, and having it be discovered that the alt-universe has nothing to do with time travel, but is just a universe that exists for some non-time travel reason, e.g. the death of Jacob as you suggest. I'm all for that. I'm just too much of a pessimist to predict that they're going to do something that interesting.
