Hmmm. Correct me if I'm wrong.
So basically. In order to Time-Travel, there are a few rules.
- You need to discard your flesh. As the body ages through but the heart does not I think.
- You need to exist in the time you are travelling to. So you can't go back to before you were born or after your current life.
- You eventually need to return back to the time you came from. You will have no memories of what you did in the other timeline.
When Apprentice Xehanort opened the keyhole to RG and split into the Heartless ; Ansem SoD and the Nobody; Xemnas, the Heartless did not have a body and therefore was able to Time-Travel.
His goal was to go back to the past and speak to Young Xehanort and inform him of the mission to gather the 13 darknesses in the realm of Sleep for 3D, as well as give him the knowledge of Time-Travel.
Then SoD returned back to the current time, which was KH1. He forgot what he did but he knew his mission. SoD's mission was to extinguish the Light of the Seven Princesses so that Darkness could fill the Realm Of Light. He possessed Riku's body so he could experience what he was creating.
Xemnas, on the other hand, was preparing 13 vessels to be filled with fragments of Xehanort. If they knew that they were able to grow back hearts then his plan would fail. So Xigbar and Xemnas lied to the other members, saying that they will only get their hearts back if they completed the Artificial Kingdom Hearts. Once it was completed then he would fill the other members with Xehanort Fragments. But that failed because some of them had already grown hearts, was too weak or was a traitor. So the only ones who had a fragment of Xehanort in them was Xigbar and Saix.
Riku was meant to be the 13th vessel but he developed a tolerance to the Darkness. Roxas was next but he had already grown a heart and returned back to Sora. So Sora became a last resort.
(Also the fact that his heart links to other character was probably a plus for Xehanort. )
That about it?