My main complaints are that I don't support attacking unprovoked and that I had expected more...radical content from the first sentence, not just details.
The radical content is the speed. Speed makes much of it work, and you can't rely on any of this working after time has passed. It is not supplementary to a slow cautious approach.
Owen is not officially the Count except by his own proclamation and de facto residence in the keep. He has yet to have his fealty accepted and he is known as a potential murderer. This is not rebellion against a true lord. If anyone continues to view this as a rebellion where we are to be faulted, let him state the reasons that Owen has legitimacy. An accepted vow? The love and acclaim of the people? An untarnished reputation for justice? He's de facto Count only because he is in Curbiston with supporters.
Now, if we wait for all those things that satisfy legal requirements, there's a larger chance that Owen finds other powerful allies outside the city which is filled with dissent. He brings those inside. The guards are changed. Everything that you suppose is a supplement to the plan of moving slowly is suddenly voided by, say, Count Zander finding a political advantage in backing his old household knight and sending reinforcements from his county. After a certain critical mass of outside help, standing behind them with swords drawn, even wobbly insiders will stand firm.
That is a pretty good plan, but isn't the dude we just got the captain of the House guard, not the cities guard?
Marshal is a very big office, effectively the control of all military. He would have supervised county-wide military details, logistics, and preparedness for most of the Count's soldiers, and also coordinated with vassals. A separate official would have kept the watch schedules and the day-to-day orders of the city guard. Probably reporting to Uriel, but hierarchies can vary. All this has been switched up by now, anyway. The key here is merely that Uriel is known by the guard as one of them. After years of being vaguely somewhere important above them in the hierarchy, he has influence with them. So we should debrief him on possibilities, and then act with an eye toward speed.
If he's coming to us, either he has an exquisite sense of justice, or his office is being removed or rumours of such are floating. Without his old office, his other holdings may classify him as just a minor knight. Or he may have large holdings outside the city. Unknown.
In any case, this war becomes his now. The former marshal leading the claim is far better than us being out in front. And he keeps his office. We say something like, "I have this boy in my keeping, true, but I have been waiting for someone of rank and importance with a sense of justice to seek him out. If you come to lead his claim, I pledge the full support and co-operation of my town and its soldiers to dispose of as you will, Lord Marshal." This is regardless whether we move fast or slow.