In summary, Gaius is a vapid monster who is wrong about evrything because he was raised in an environment designed to create vapid monsters who are wrong about everything.
It just occurred to me that the Scions' fear of Imperial genocide after learning about Shiva was entirely pointless, as Gaius already knew. It makes up a decent chunk of his Praetorium monologue.
Thordan ran away from Ishgard to acquire the power to fulfill his scheme, giving the Scions time to turn the entire city against him. There's no way the Populares will entertain the complete rebuild necessary to root out Ascian influence and let Garlemald stay a world power.