You must read all of the comments as well, to understand fully.
LA X just started making sense to me. The time lines did in fact intersect on the flight to LA and was caused by Desmond not pushing the button. The X is an Easter egg from the writers\producers. This may have been covered here before, if it has I applaud whomever thought of it because it took me this long to catch on.
The timelines crossed or ran together for a brief moment during the Electromagnetism Energy Release over the island causing the flights from the ALT TL and the OT to be in the very same space at a single point in time by taring a hole in the fabric of space and time which immediately closed back up.
What an Easter Egg huh! Has this been mentioned before. I have just always thought that they meant the Airport in LA i.e. LAX but spelled it like LA X. That is great. Patting myself on the back.
So IMO the bomb did not detonate. They have not changed their own reality but a hole was ripped in space and time for a brief moment giving their Alt selves a peak into their timeline of events because Desmond did not push the button while they were over the island.
It was the original error of Desmond not pushing the button that caused the two parrallel worlds to intersect over the island. The bomb did not go off. The wheel was turned again by someone (my guess is Ellie) sending them to their correct time once more.
Desmonds sacrifice will be to not forget to push the button and letting Kelvin Inman leave on his boat.