Well this may be semantics, but the Island wouldn't have sunk unless 815 crashed...Of course they diverged before the 815 crash. They diverged at the time that the Island sunk.

The real point is that we don't know when the island sunk in the alternate reality. We can't assume it happened in the '70s (and it doesn't seem possible anyway) so that helps with the logic.
Meaning, the blast in one reality did not have any impact on the other reality, and Jacob has the ability to move across realities and change his behavior so that the realities diverge? Jacob, not the blast, is behind it all?I am now currently convinced that the flash-sideways aren't what we think we're seeing. I think that they're the lives the Losties are leading without Jacob's interference.
It would be weird if the blast had no impact on what we're seeing. We have no evidence of it, but Juliet's "It worked" tells me that the writers want us to believe the blast had an impact.
She mentioned that them, her parents and Locke's father should just go to Vegas. What the hell? Are they on good terms? How did Locke become paralysed?
That's an easy plotline to whomp up. Locke was raised by his dad. At some point, dad was driving when there was a car accident that paralyzed Locke. Karma's a bitch!