He just wanted to rule Liberl :(
It's usual Campanella shenanigans, lol. I would like to think of him breaking the fourth-wall. Fits more with his whimsical nature.
I mean, no one really expected Osborne to be alive and kicking after taking a bullet to the chest. Well, Vita mentions Ouroboros suspecting that 'death' wouldn't stop Osborne from clawing his way back. He's indeed pressuring Rean hard, to work with him. Sucks that Rean doesn't really have much choice, since, he's still incredibly curious what's going on with his chest and familial background. And, don't forget the Church, they're definitely up to something! We know of two Dominions operating within Erebonia, and I believe, Kevin and Ries never went back to Alteria after they finished up in Crossbell. So, possibly three Dominion joining in on the fray.
I've heard people mentioned Calvard might having Earth, no idea where the source for that came from though. Crossbell's Sept-Terion, is actually Mirage, which fills in the Phantasmal part of Phantasmal Blaze, the Azure Demiourgos just had the added effect of being able to control Time and Space as well. Actually, Erebonia has two Sept=Terion in their borders, one is presumably Fire because of Phantasmal Blaze, and the other might be Time or Water; depending on how you would interpret some of the hints dropped. It's highly implied that the second Sept-Terion was gained through Osborne's little annexation spree he did, so, it might mean that he's in possession of it.
I want to say that the series doesn't really acknowledge the Star Doors being 'seen' by any of the characters. Of course, there never was a point of time where the that knowledge might come into play, yet. To be honest, Joshua and Estella deserve at least S-rank, lol. That shit was a national crisis, and they solved it almost perfectly... Only for Cassius to come swooping through with Ragnard like a badass, lol.
Definitely. And, with a name like the Blood and Iron Chancellor. Osborne capitalized on that opportunity big time.