I can answer at least some of these!
-He can't die unless you destroy the Heart of Chaos within him
But he can die! Just not permanently. That bahamut technique he does kills the user, he just gets "reborn" instantly.
-The Heart of Chaos and role of Guardian can only be passed from Caius to another if they kill him -- but they can't kill him unless they destroy the Heart of Chaos, in which case there's nothing to pass on anyway
I thought the way Noel explained it was that you don't really destroy the heart, you consume it and kill the user. Caius did this to his predecessor, and Noel was able to do this to Caius too.
-An ordinary sword was sufficient to pierce the Heart of Chaos, but burying Caius under rubble or blasting him to hell does nothing
Well, 1) only the next guardian in line could pierce the heart and 2) why would knocking him around pierce the heart?
In the end, even destroying the Heart of Chaos doesn't keep him dead (but this may have been because he died in Valhalla)
Yeah, he died/released Valhalla, a place with no time where the dead are supposed to wander. So he's not really dead but he's not exactly alive either, imo.
He ends up doing it himself anyway; Noel is just holding the other end of the weapon, but applies no force of his own, and has no desire to kill him
My confusion about this mostly stems from the fact that the Paddra seers tell Noel he has to be ready to kill Caius, and answering yes is the only way to get all the fragments.
However, maybe that's exactly why shit got fucked up. If Noel had it in his heart to kill him, being the next guardian that conviction would have "blessed" him with the heart. If it's all related to the will of the potential guardian when they pierce the heart, like they imply, maybe Noel's hesitation to kill anyone was exactly why Caius believed he could destroy the heart and have no one inherit it.
babble babble