Timeless River does go back in time that was why maleficent said that "on the other side of this portal the're about to begin construction on that retched castle" and Merlin said that the nature of the world may tempt them to do something dark. But more on topic, the door to Timeless River was special Merlin Magic, young Xehenort didn't have one so in order to time travel through darkness it was neccessary to cast off his flesh.
Timeless River is a world made by magic. It's called Timeless River because time doesn't go on, it's endless. It is based in the past though but don't confuse going there with time travel. The two are different. If Time Travel was as easy as Merlin using magic like that, then he could easily stop the events of Kingdom Hearts.
Young Xehanort mission was to escort other norts from past timelines together by the end of DDD wans't it? And once Xemnas was there, a Nobody, that means a body, flesh, time traveled. That was confusing, I just wan't to see theories about it, it's impossible to be 100% sure right now, it's like you say me you understand Evangelions ending. Kingdom Hearts went Gainax with the nonsense time travel.
Hm...I think we might need to wait for better subtitles but yeah, I see where you're going with it now.