A Theory: Master Xehonart takes over Terra's Body, thus explaining MX's Youth, and why Xehonart looks like Terra. When Xehonart Becomes a Heartless, the body, Xemnas, becomes Terra again, explaining why he knows of Aqua and Ven. While The heartless is MX, thus explaining his appearence. The lingering sentiment is Terra's Memories/Soul that was pushed out by MX.