I believe future Simon is a little older than he leads on. Even older than the audience realizes. His statement to Alisha that "there are things that need to happen" leads me to believe he's rewound time several times at various times to 'get it right' so-to-speak. The pix on the wall cover timelines extinct because of rewinding - Vegas may be one of them. Like Curtis, he's immune to the effects of the time jumps. (he didn't need the extra power of immunity for that). I do not think that his final save was neither "seen" nor avoidable because he's not able to rewind time.
** just saw simon's death episode and have to recant part of my above statement... Keeping with the BBC theory that there are fixed moments in time that must happen (the world wars, certain musicians, poets and politicians existences etc.) then:
I believe future Simon is a little older than he leads on. Even older than the audience realizes. His statement to Alisha that "there are things that need to happen" leads me to believe he's rewound time several times at various times to 'get it right' so-to-speak. The pix on the wall cover timelines extinct because of rewinding - Vegas may be one of them. Like Curtis, he's immune to the effects of the time jumps. (he didn't need the extra power of immunity for that). there is only one simon. his death, to me, proves that in itself. he does cross over his own timeline but only inasmuch to keep the time line HE knows to be the "correct timeline" from already living it. when things go awry, he "fixes" the timeline to proceed forward to his enviable death. that's also why tere are so many photos of the crew that we do not recognise. the audience is not shown those tmelines, just the "correct" or main one.