So, okay, the plans are fine, as is. Here's an alternative to stir things up a bit.
I'd point out that a veteran of the old guard like Uriel can probably bring a lot of people from inside Curbiston with him. There's a lot of attention so far on starting the ball rolling outside the city among other knights and the Duke, and there are suggestions about riots inside Curbiston, but Uriel himself being here suggests the support of the garrison and men-at-arms is also weak. We can debrief him on this matter, and find out how soft the city is right now.
Owen probably hasn't had time to purge their ranks, and if we wanted to, we might be able to boldly move on Curbiston with just Uriel and our mounted dragoons alone. Tomorrow. Uriel can persuade the gate guards to let us through, we're in the city, we garrison the gatehouse and move toward the Keep blaring out with horns and cries of "For the true Count! Down with the murderer! Down with Sir Owen!"
Uriel gets half or more of the city's own guard to aid us, the common folk get their staves and clubs, and behind this meatshield, our own archers are sniping whatever idiot peeks out a battlement inside the keep. Instead of sieging a city, we're down to sieging the keep with much of the garrison behind us rather than across from us. This looks extremely hopeless from Owen's standpoint.
Curbiston falls largely intact with forces mainly from within.
On the other hand, if we view this entirely as a matter of those outside opposing those inside, moving deliberately and slowly to put Curbiston under siege, then we corner the insiders into reluctantly supporting Owen.
Give it a think.
That is a pretty good plan, but isn't the dude we just got the captain of the House guard, not the cities guard?