Can we throw in a little Slaughterhouse Five parallel here? Time is not a line, time is a circle. Everything that happens, has always happened, and will always happen. The characters are not torn between two universes (the island and the non-crashing plane); they are alive in two different periods of time (as Billy Pilgrim frequently time-traveled). The only thing that alters this "destiny" is the human ability of free will.
Take for example Julliet. She always fell in that hole with the hydrogen bomb. She always fell in there and always died. However this time, her free will instructs her differently; her free will chooses to smash the bomb and blow up the island. Why is this time different? Because this time (1974), Sawyer has joined her (from a previous decision made by his free will). Sawyer and her fell in love, they shared an amazing bond; she knew that her and Sawyer both believed in the same thing (as told by Jack that they needed to erupt the H-bomb) and when she was down there, she took the initiative to make that happen! For her, for Sawyer.
As for Jacob and Esua, I feel like they are merely spirits that body jump to remain within a physical entity. The bodies that we see Jacob and Esua possessing in (with the beards and robes) aren't their birth bodies. Rather, these are the last bodies that the spirits were able to jump into.