This is a theory I came up with back during all the Little Ben drama in "He's Our You" and "Whatever Happened Happened" and so on. It's kind of like a hybrid between the Whatever Happened Happened and Changing the Timeline theories.
Maybe events in the past can't be changed or erased, but they can be altered slightly. In "Dead is Dead," Ben told UnLocke that he was healed in the temple, so we know it must have happened all along. It's possible that Ben always had a near-death experience that led to his healing, and Sayid just changed the way it happened, similarly to how Desmond changed the circumstances of Charlie's death. Because of Course Correction, "what happened" will always remain the same, but how something happened can be changed. For example, "Charlie died because he got struck by lightning" could be changed to "Charlie died because he drowned in the Looking Glass station," but the fact that "Charlie died" remains constant.
Daniel said that the past cannot be changed. Maybe what he meant is that the Losties can't control the destinies of the people living in the past (DHARMA, the hostiles, etc.). However, they can control their own destinies because 1977 is their present. Anything they do in 1977 can affect them because they have control over their future. In my opinion, the future is not completely set in stone and they can change it based on the decisions they make (Again, Desmond changing the way Charlie died).
If you apply this theory to the bomb...it means that the circumstances surrounding the incident can be altered, but the overall timeline can't be changed. In other words, even if the Losties succeeded in changing some things, the universe will course correct and make sure Flight 815 gets to the island. Everything that is supposed to happen will happen somehow. Does that make sense?
If anyone has any ideas that can disprove this theory, please post! Now that I think about it more, it kind of does have some holes in it, but I think it's still a possibility.