It seems to me, that Xehanort was indeed directly connected to the keyblade war. If you look at AF1 when he takes off his helmet, you can see that it looks like a younger Xehanort(non heartless). He has the same wierd slightly pointed chin, same eyebrows, and the same pointy middle part of his hair. His hair has not turned white yet. Although the hairstyles are slightly different, it is possible to change one's hairstyle over time (obviously). Since he was found without a trace of memory, he could have easily fought in the keyblade war(Also, Xigabar directly said that the two events are related(Finding him and the Keyblade war))
As for Zexion and Xigabar's conversation, it seems to me that the room of awakening, since in Castle oblivion must be the room were Sora goes to sleep then wakes up in the giant flower thing. While inside the thing, Naminé was able to rearange Sora's thoughts. Since Xehanort had memory issues(forgetting who he was) it is safe to assume that Xemas has them too. He may be searching for a way to get his memory back in castle Oblivion(which is closely linked to memory. When Xigabar said his other friend might be there it could have ment the memory of his other friend. Also, the point of the armour being in that room could be so Xemnas can remember that person(Aqua(the girl knight)). So whenever he goes down there, it would be like visiting an old friend, since it is the only time he remembers her. Additionaly, Xigabar saying that it was the one Xemnas didn't build implies that Ansem the wise (or as i like to call him, Ansem the White). Meaning that Ansem the White may be directly connected to Castle Oblivion.
Going back to Xehanort's memory loss, it would also explain why he went around calling himself Ansem. After getting rid of Ansem the White, he could have lost his memory again, looked around, saw he was in charge and alot of stuff with ansem's name on it, and assumed thats who he was.
Those are just some thoughts. What do you guys think?