let me preface this with, "its just a thought."
what if "whatever happened, happened" was a true statement, but the losties in 77' did in fact change the future?
WHH; all the events that happened that brought the losties to 77' happened, they can't be changed. they crashed in 2004, they were rescued, they came back...ect. They are in the 70's that cannot be changed. example: you go back in time and kill your father, you still exist because the events that brought you there happened, Whatever happened, happened.
blowing up the bomb; the losties did blow up the bomb, only the effects of it only exist in the 2004 that is going to happen, not the 2004 crash that brought them here. example: here is a time line to help explain.
1977------2004 815 Crash--------Rescue-------316/1977 FLASH------JUGHEAD----------2004 815 Takes off for LA-----TBA----
does that make sense?
As for Sun that that time line... well Jack, Kate, Hurley and them are just gone. They no longer exist in her time line. I think the jumps instead of being flashbacks or flash forwards, or time jumps it will be time line jumps. The new one created after the bomb explodes and the one Sun is in.
I believe the end resolve for the show will be to bring us back to the first season in a loop that is double sided, or in some way try and merge the two time lines together. explaining who adam and eve were, how the food drop came out of no where, along with everything else that seemed out of place.