There is also another thing to consider. The 13th is flooded with darkness, which could result in a stronger version of Zodiarks essence even though it's just a part of Zodiark. Maybe irrelevant but food for thought.
Golbez is a walking wild card, the devs could pull all sorts of stuff with him in the last patch(es). I wonder if they resolve some questions you have with the Ascians?

The suspicion about his past is there. They went back to the King and probably got ambushed or betrayed by the King.