You think Xigbar/Braig is MX? He's not, MX is Rihanna! Duh. That song Unfaithful? 'Bout Kairi's Grandmother, just sayin'.
Anyway, it's already been stated Braig saw TAV when he was an apprentice with AtW, so I think that kills Braig/Xigbar being MX. Especially since Braig has no Keyblade (most likely) and isn't old.