Hey I think Terra is really Xehanort cuz check it out they look the same. I mean think about it, why else would xemnas call aquas armor 'friend'. And maybe since terra is trying to save vens heart when he turns into zemnas, he try's to get his aquas and vens heart back by summoning kingdom hearts. I think something happens after the MX battle and terra looses his memory and somehow changes his looks. And xemnas probly loves to have roxas in the organization and has everyone looking for him like crazy when he runs away because he reminds him of ven. And since xehanort lost his memories, maybe he awakened them when he lost his heart. Maybe thats why xehanort volunteered for the experiment ansem was gona do. Because he somehow knew the experiment would somehow make him remember his past memories and he wanted that the most. i think thats why he studied under ansem.