When Mark burned the friendship bracelet instead of putting on the board, he changed his future. The episode works very well as a character driven episode, about Al's choice of how to deal with his responsibility for that woman's death, and how he justified it.
But, making the flashforwards into cautionary tales instead of simple foreknowledge, adds a contingent element. Even if it doesn't turn out to be "God" who is gave the divine warnings to change everyone's life, whatever plot element turns out to be responsible will enact the same role. The flashforwards are a paradoxical glimpse of a future in which people saw a future.
The insistence that what the characters do doesn't matter unless they can, basically, "win" against destiny, is childish. Free will is the ability to do what you choose, not in determining the outcome of complex events. Saying otherwise is like saying you only have a free vote if your candidate wins.
In practice, ignorance of consequences is a major element constraining free will. The flashforwards, if they had been a vision of "the" future would have posed real choices for free will because they offered the extra freedom of knowledge. And choice is the essence of drama. (Winning and losing is the essence of melodrama, I think.)
The show could have been like the fable about Death's appointment in Samarra. But turning the flashforwards into divine interventions (in essence, even if not formally) turns it more into The Bridge of San Luis Rey, where we see how it all works out for the best (i.e., the "heroes" win.)
