To start, I have to say, this is by far the most confusing plot I have ever experienced. I have played all the other KH games and understood everything just fine, with perhaps a few question marks over my head here and there, but this is the first one that has left me entirely perplexed as to what even happened. That said, the reason I am asking about the dream worlds and time travel in the same question is that, as far as I can surmise, the two concepts seem to be fairly intertwined. But of course, I could be entirely wrong.
So, as far as I can gather, Sora and Riku have been sent to the realm of sleep, where they are to unlock the sleeping keyholes and awaken the worlds from their sleep. The worlds that they experience are the worlds' dreams. But the Xehanort trio (young Xehanort, Xehanort's Heartless, and Xemnas) managed to enter the dream world along with them, sabotage their mission, and set their plan into motion.
But here's what I don't get.
1. Goofy implied that Sora and Riku jumped through time in order to enter the realm of sleep. If that's the case, what exactly is the realm of sleep, and why did they have to travel through time in order to enter it?
2. How did the Xehanort trio enter the dream world along with them? Goofy says that he traveled through time in the the same way as Sora and Riku, and Master Yen Sid and Mickey say that he was able to do it because Xehanort's Heartless was controlling Riku. Why does that allow him to enter the dream world??
3. Finally, Master Yen Sid said that you have to abandon your body to travel through time. But if that's the case, how does teenage Xehanort travel through time freely with his body still perfectly intact?
I apologize if my questions sound all over the place. But that's really a reflection of how perplexed I am by the game's plot. If one or more of my questions needs to be clarified, please let me know in the comments and I'll edit accordingly.