I don't think that Aqua really dies in BBS, at least her heart being taken away or just memories asleep..since she still can talk with Xemnas in Room of Sleep...
Mickey found the keyblade in realm of darkness..
I don't think she dies either. I mean we never really see what happens to her.
As far as Xemnas goes, I'm still surrounded by the fact that he found Aqua unconsious and brought her down to the basement of Radiant Garden and placed her in resuscitation and when the time was right, he went back down when as she was awakening.
What I don't get is that if Xigbar hears Xemnas talking to somebody when he's in that Room and "if" Aqua is in resuscitation, who is he talking to?
And right, King Mickey found The Inverse Kingdom Key in the Realm of Darkness.