LAST EDITED ON Jun-03-15 AT 08:14 PM (EDT)
You have a point with the Matalde-angle, I'll give you that. But here's the thing I'm betting that Akio's banking on, other than having Annabelle and Garnet as bargaining chips against the Duelists; Oriphos is its own unique realm, separate from the whole of creation and complete unto itself, like Cephiro. Here's the thing:
It took the Chief going to some fairly extensive lengths to just REACH the realm when he was trying to rescue Raven from her father. Admittedly, UF-Gryph isn't a divine anything, but he knows quite a few comparable beings of the appropriate power level that probably COULD have gotten him there, even in the relatively small window of time he had had in which to pull it off. I'm guessing that Akio is sorta/kinda the Pillar/Godking/whatever of Oriphos and thus can completely block (or nearly so) all avenues of access to his realm if he so chooses. If not, then he has access to such means, and no-doubt plugged every hole he could before planning the smash and grab. I highly doubt that with his level of access to arcanum and whatnot he failed to figure out HOW UF-Gryph got into his realm the first time, after the fact, and so I doubt that method ingress is available anymore.
Of course Utena, Corwin, and the rest are no slouches in the "raw brute force when necessary" department, but I just wanna point out that if they went that route they could potentially kill everyone in Oriphos with it, infant and hatchling included, never mind the hundreds of students attending Akio's Academy. Brute force approaches like that may be good for bank vaults, but only if you don't care if some of the contents of said vault are damaged, in not destroyed, in the attempt. And you rarely get the choice of what happens to the items within in such a scenario. And I doubt that even in their aggrieved state that the Trinity and the Duelists would go with the "fuck everyone and everything and get our kids back NOW" approach; they're not the the type to do it.
After all, how many people down the years have they seen, who ultimately managed to turn their lives around and redeem themselves after some initial bad choices that once put them at odds with the Duelists? I can't see any of them going scorched-earth mode with Oriphos if there's even a chance that they could redeem some of the inhabitants.
In the end, Akio doesn't have just two hostages, he's got throngs of human shields from all over the realms to put between himself and his foes, and if someone from the Black Rose prisoners they have after the assault crumbled were to make it known to the Trinity and the Duelists (Saionji and Anakin are the most likely to bring it up, but I'm not discounting Leyna Taggart here either...), I'm pretty sure that they'd agree that going in and cutting everyone down wouldn't be an option. That'd make them no better than Akio himself.