How does Simon remember his destiny?

Whenever any TV Series/Movie employs time travel there tends to be some major gaps.

Unless I'm missing something there are several in Misfits throughout the first three series, mainly the latter two - and yes, I think they must just be plot holes - I'll give another example or two.

A major one is Superhoodie's actions in general - if he came back to save Alisha, why does he need to act upon so many different things? Saving Nathan, Curtis etc. It suggests that there have been many versions of time in which Superhoodie has slowly improved the timeline with each attempt, but it makes absolutely no sense.

DO NO READ ON IF YOU HAVEN'T WATCHED THE THIRD SERIES YET!

It turns out the time arc is complete when Simon heads back into the past at the end of the third series. His only reason for going back is to see Alisha - not save her - or he'd simply stop her throat being slit and find a different way of stopping her being shot by the gamer bloke.

Let's rewind a bit. If he went back solely for Alisha, what's with the other actions? For example, he saved Nathan from the Virgin group at the end of series one - but why is that necessary?

None of series two or three would even have happened if Nathan had been converted by the group, so surely the original timeline would have been all but Simon being converted and a totally different world and story being created. I'm rambling but you get the idea.

A simpler example is the Nazi episode. A whole different reality is created. In that world, Kelly would have grown up in a totally different way, under Nazi control etc.. she would not have lived the same life. So how is it that when she travels back in time at the end of the episode and stops it all from happening - she is able to go back to the normal reality and speak to the rest of the gang as though she never left? It makes no sense whatsoever - the non-Nazi world should be totally alien to her.

Probably a more elaborate answer than you were hoping for and one that probably raises more questions than it answers. Sorry!