And another thing, if Terra is all fine and dandy and in full form in Keyblade Graveyard, why would he just be chilling there atm? You mean to tell me the guy who was so fixated on defeating Xehanort that he made his armor come to life just to do that along with reuniting with his friends Aqua and Ven and keeping them safe.... you mean to tell me a guy like that would just sit on his ass right now? Yeah again, I'm not buying it especially seeing how Lea went about his business after getting recompleted.
At this point I'm kinda buying the vessel theory but crossing my fingers for Dark Realm/Keyblade Graveyard. For arguments sake, here's some reasons why Terra would be chillin' out at KG.
One is that travel between worlds is hard. In KH1 the only reason it worked is because the heartless were tearing down the walls. The only reason it worked in KHII is because Sora was opening pathways to specific worlds, and could only travel to worlds he opened. It's been shown that you can use the darkness to teleport around, but without a fancy black coat its effects are harmful, and I don't think its been proven that the armor TAV had was darkness proof. This was the only reason Lea was able to go "about his buisiness", he used darkness to get around and had a black coat. TAV also had components linked to their that let them travel, and it looked like the darkness portals but light-based. His armor would still have that component, but who knows if it still works after 10-15 years or however long it is, or if Terra could even claim the armor IF he reformed at KG since it's moving of its own volition, or if that method of travel even still works after all the stuff Xehanort's done to the worlds.
A second point is that Terra pretty much should be absolutely ashamed of what he's caused, so he'd probably be really apprehensive to find people he affected because he felt guily. Kinda like how Riku felt when he asked Sora at the end of KHII "How can I face everyone after what I've done?", except that Terra doesn't have a goober to make fish faces at him to cheer him up. He probably has full cognition of what Xehanort used his body for, and here's a short list: fought Aqua, was responsible for Aqua's abandonment in the Realm of Darnkess, attempted to access Kingdom Hearts to cover the World in darkness(KH1), attempted to start the Keyblade War multiple times(KHII and DDD), drove Riku to Darkness, and fought Sora 50 billion times. Terra was indirectly responsible for all of this, so I imagine he'll have some weight on his conscience when/if he shows up.
A third point is that he would be scared of Xehanort. Not in the sense that he's actually afraid of him, but scared of what he could make him do to his friends and the present Keyblade wielders. Because he's already fallen to him once, and nothing has shown he wouldn't just be susceptible again.
So yeah. If he was at KG, its possible he can't leave and that he wouldn't want to.