The trouble is with time travel is that a) it's pure fiction b) regardless of how you spin it it can never make any sense.
Fundamentally, this is the problem...
If you send someone back in time and his decisions affect your (future/present) reality, then your own reality is now in an indeterminate state (the writer now has to decide what happens to the future/present by making up his own rules, which he'll probably break anyhow). If you send someone back in time and it simply causes a so-called "branch" in reality, then there's no point doing it in the first place.
All in all, stories that rely on time travel as a central plot device never stand up to any kind of scrutiny. Because time travel cannot be possible without creating so many paradoxes that it would fundamentally undo the universe.
That's assuming what scientists THINK happens with time travel is correct, they're a long way off proving anything. In all likelihood it'll always be theoretical, at least in our lifetimes.