I almost wish I'd had a proper scheme to oppose Hireshmont because that would've been fun. There's not really a 'Quintus' side' in this that I can see, though. An army took over his castle and he promised to behave. If Terran falls, it won't be because Quintus didn't fight to the last man.
In other words, even if Quintus does do something to defy Hireshmont's plans (whatever those plans are), I'm not sure I see how it could be a strong enough move to make a difference. At best he could save Aurvandil a week of effort.
I'm all in favor of intrigues and there were some discussed a while back, but they didn't happen for various reasons. The 'senate trial' seems like a weirdly-timed power play against someone who just lost the power he had. So Hireshmont could ban Quintus...Aurvandil takes the Chateau. Hireshmont doesn't ban Quintus...Aurvandil takes the Chateau. If Vellos has a rabbit to pull out of his hat, Quintus being Duke or not or banned or not can hardly make a difference.
It's not a power play.
Hireshmont, as you note, has nothing to gain here: and any chance he may have had of securing personal mercy for himself (which was basically impossible) will certainly be ruined by punishing the one noble in Terran who has thus far taken such an option.
What Hireshmont is interested in is making sure that there is a republic of Terran after all of this: and not just a realm called Terran. He's asserting the superiority of the Senate and the Republic over any individual Senator.
Here's another way of thinking about it:
When I created Hireshmont II, I did so intending to play a very idealistic character who would be strictly law-and-order all the way, and deeply committed to a lofty ideal of nobility. The polar opposite of his father, who was an opportunist and pragmatist who would do anything he had to in order to achieve his (sometimes dubious) objectives.
Hireshmont is the kind of person who doesn't speed on the highway even if there aren't cops because its the Law and the Law is the manifestation of the Good. And when he brings a legal suit, he does so because its the Law– you don't have to think any more about reasons. When a lord negotiates without Senatorial permission, he attacks that lord. When Rynn negotiates with Luria without working alongside the Moot, he attacks Rynn. When a Chief Magistrate sticks his toe over the legal line, he attacks him. In Terran's early days, when a series of Generals tried to veto dukes' picks for marshals, Hireshmont the duke attacked them. He's an obsessive treaty-writer, law-giver, and idealist about the nature of order and law. If you think back on Hireshmont that interpretation will explain most everything he does. And it explains the legal suit now. Quintus broke the law. Even if breaking the law saved Terran, Quintus has got to pay, because a Terran without the law is a Terran better off dead.
