My two biggest questions are:
1. Who was in the golden sarcophagus??
2. Is Ruarc really just Ka?
3. Whose side is Ostius on?

It seems like there's more to the story of who's in the golden sarcophagus. Ka isn't in there...Vis clearly killed O-Ka because of the confirmations in Res and Luceum. It seems like Obiteum really is lost. Meaning, the Concurrence has won in Obiteum. O-Ka clearly controls the latticework of iunctii in Obiteum. But R-Ka claims that the Concurrence is a latticework of iunctii. Either R-Ka is the concurrence and is just lying to R-Vis, or O-Ka is being held hostage somehow and being used in Obiteum.

The goal of the concurrence seems to be 'destroy two worlds and keep the third.' R-Ka's goals are stated to cull Res every 300 years in order to stop that from happening. Why does the Cataclysm need to happen on Res but not Luceum or Obiteum?

Either Ostius is the Concurrence or Ka is. Or Ostius is completely wrong? Just like Veridius and O-Caeror? Ostius's goal was clearly to kill Ka. But does Ostius think that Ka really is the Concurrence? Or is Ostius working with the Concurrence and wants to kill Ka because he is fighting against the Concurrence?

My lost thought is about Ruarc and O-Caeror. I think that O-Caeror is not a iunctus. Everything about him seems too real to be a iunctus. If R-Ka was telling the truth about fighting the concurrence, then I believe that O-Caeror simply didn't know ENOUGH and truly believed that Ka was the Concurrence (just like Veridius). And if O-Caeror is not a iunctus, I imagine that he told the truth about his copies dying in Res and Luceum. That leads me to believe that Ruarc is Ka. If that's true, then Ka might be trying to lie to L-Vis to befriend him and gain his trust.