So the way time travel works in this universe is, a version of yourself needs to be at that point in time. That's how Xehanort was able to find Ansem and Xemnas. However, in Kingdom hearts 2, Sora is able to travel to the timeless river which is arguably the past. There was no version of Sora. I'm sure different means were used to travel through time by Sora and Xehanort but that still goes against that rule. Explanation? Thoughts?
You do have to keep in mind that KH2 was written LONG before time travel was even a twinkle in Nomura's eye. Pretty much every mystery or inconsistency in this series can be answered with the same explanation: Nomura hasn't thought through half the stuff he writes.
The excuse we're going by now is basically that the time travel method Xehanort used is kind of like the universal standard, at least for Keyblade wielders if not for everybody, but particular people in particular worlds can occasionally have unique time travel methods that use rules contradictory to this standardized system, and really the only reason for that is because it's consistent with the movie the character(s) and their world(s) came from. So, Sora and Xehanort and people who originate in this series have to use the standard method of time travel because it's the only one available to them. People like Merlin can use custom modes of time travel because the nature of their world or the nature of their specific background allows for them to use it. Basically, if you happen to come from a world that allows for its own system of time travel, or if by chance you happen to be a character with exclusive knowledge and capabilities to use a certain unorthodox means of time travel, then you can buck the standard system and have at it. I don't know if this would ever apply to Final Fantasy characters though, as even though several have background experience with time travel (i.e., Ultimecia from FFVIII), I have no idea if they are more closely bound to the limitations of the KH universe because of the way they're all effectively severed from their original stories, worlds, and backgrounds and rewritten in this series.