Guys, I want to point out that when Terranort appears in RG, he's controlled by MX. He has the yellow eyes, and says he's Xehanort.
But in the secret ending, he has longer hair (so that means several months passed), Braig calls him Master.
So why do you think ? When Terranort first showed up, he had Terra's exact HAIR and worst, CLOTH, and Aqua's armor, and keyblade ! There is NO WAY Braig hadn't already figured out that it was, at least, Terra's body.
So if several months later, when the yellow eyes (which represent darkness) disappeared, Braig is surprised by looking at him, and recognizes Terra (while calling him "Master" for Master Xehanort before), I think it's safe to say that Terra has regain some control in here. Or at least, prevent MX to...
Terra says he doesn't care if he becomes darkness, he has only one goal...
What if Terra actually TURNS dark to accomplish his goal (save Ven and Terra maybe ?) ? I just don't see MX here the whole time while Terra and Eraqus' heart are inside the body.