The problem with time-travel...Very confusing.
However, I think it's safe to assume that perhaps Hiro's power isn't actually time-travel.
What if, Hiro's power would actually be that as trans-dimentional jumping? Allow me to explain.
Prolbem: Future Hiro created time lines, that dictated that if Sylar was stopped from taking Claire's ability, New York would be safe.
Solution: Future Hiro travelled back, and told Peter to save Claire, which he did. However, when Hiro returned to his time, the world remained unchanged.
Problem: In Future Hiro's time, Past Hiro stabbed Sylar but he regenerated.
Solution: Since Hiro stopped Sylar from getting regeneration abilities, this should have prevented Future Hiro's timeline from occuring.
Ultimate Solution: Future Hiro is actually a trans-dimentional Hiro, the result of different potential outcomes from the entire year.
As such, there would be Hiros that saw Sylar defeated, however saw the world continue down it's dark path. Or ones that saw Sylar victorious over the other heroes.
Considering alternative dimentions are the only thing that allow continuity to exist with the Five Years Gone episode, that has to be the solution. Since Claire was alive in Five Years Gone, Hiro couldn't have stabbed Sylar in New York and have him regenerate, considering Sylar was also alive in Five Years Gone. One of these two characters HAD to die.
Also, notice Bennet's surprise when Ando tells him that Claire is alive. It's as if Claire wasn't killed at Homecoming, possibily due to Bennet and his team (Eden and the Haitian) stopping Sylar after he'd killed Jackie. Knowing what Claire could do, and not to mention what his Company would do, Bennet falsified Claire's death and hid her from Primatech and Her Family.
It's one possibility or the other people.
Therefore, the dimentions are not changable. The only current changable dimention would be the CURRENT one that we've seen for the past 19 episodes. It's events have yet to be determined, and rely on whether or not Hiro can succeed in stopping Sylar.
Of course, I don't know a damned thing about Time Travel, but this makes far more sense.
It also allows the open possibility of other questions to be answered.
Also! For anyone else interested, there's a Heroes RPG Board trying to get started located at:
http://www.got-heroes.com/Role-Playing-aka-RPG-f32.html
Join up, and let's have some fun people...And yes, I'm Mr. Bennet.