Hi, i been reading this page for a long time but i just created the account.
So here is my theory. I think Sora leaves at the end of 3D to find a way to "set roxas free", and somehow Xion as well. That is why he goes alone, because only he can do something about that.
Think about it
I dony think Nomura would make Roxas accept he cant be himsef, after making a game about him, creating one of the best characters in the series.
Axel/Lea intention is to help Roxas and Xion, even if helping Sora is "the same" he want HIS friends back, that is the whole point of he helping everyone.
Also he appeared with everyone else en 3D opening, like he will end up fighting the Norts.
Sory if my english is bad, spanish is my first language.
Hope you like it