Is it possible that Caliborn and Calliope weren’t actually the aspect that they were assigned? What I mean is- since they were the only two players, is it possible that their aspect was compromised so they they would be able to play the game?

Nope. The game gives 0 fucks about whether your Session is viable or not. Besides, Caliborn had no intention of playing a 2-Player Session, Skaia would’ve known that, and not even bothered to make the Session viable if it wanted to.