I'm aware of that, but that's something I chalk up to gameplay convenience - the amalj'aa aren't going to wait for you to come back before feeding the captives to Ifrit. Technically speaking there's nothing stopping you from leaving any of the urgent, imminent battles to go gallivant about the countryside until you're ready to fight them. Yotsuyu-as-Tsukuyomi? You actually can leave Castrum Fluminis and come back to fight her later, despite the fact doing so would narratively be somewhere between extremely stupid and downright suicidal. Vauthry will wait for you at the Crown of the Immaculate if you want to go have fun with the dwarves beforehand.
Anyway, I did forget what the Scions were doing while you were fighting Titania; I just remembered them standing with you at the base of the stairs leading to the Crown of the Immaculate. I know you can't take Trusts into trials, but because of how the story is designed (you had a hole punched in reality just for you to go to Norvrandt!) the likelihood of having adventurer buddies to help you with Titania and Innocence is again somewhere between "really low" and zero. Hades gets around this by showing Exarch G'raha explicitly summon other champions to help you (probably made easier because he was just sending out a call for anyone to pick up, instead of trying to reach a specific individual in spacetime).