I think the writers are trying to avoid issues like that because if you can just go back in time and save someone the show can never really have any conclusion as it is never over. Also you get bogged down in time parodoxes i.e.
If Peter went back in time and saved Simone she would still be alive,
If she didn't die he wouldn't have gone back and saved her so she would die,
As she died we go back to the beggining and get stuck in a loop!
So I think it's a gray area the writers are keen to avoid!
