Meh, NRS sounds like a pointless distraction now. That settlement is an embryo that can be aborted in the fulness of time; meanwhile the siege is time sensitive and comprises the bulk of enemy forces. Let's cut this off at the head quickly: ferry the army across the river and march on the real RS.
The ferrying of about 2600 soldiers will take a few days even over the width of a river, so we will go first as a reconnaissance-in-force and reprise our role of outrider in the last war with maybe 50 light cavalry and our 42 dragoons, plus a quantity of lesser-armed pure scouts and couriers. Each man, two horses and adequate field rations, though we can also forage. As we go, talk to the locals and gain intel on where to go cautiously, while telling them to rally forces to the advancing main army. The sea raiders cannot have marched their army across much of the countryside in the small time that they've been established here. We give towns notice to prepare joining the Duke or evacuating. If we run into a sizeable enemy, try to circle it and keep scouting. Let's try deep penetration up to the supply lines. There we cause enough hit-and-fade mayhem that the raiders send a detachment to crush us. The joke on everyone that we pass, and the detachment sent to crush us, is that we're not actually close to the size of the real threat that we've blinded them to.
As for the people here worrying about their ships escaping, no worries. You've got 200-300 ships beached on the shore at high-tide that take a certain amount of men to push off into the sea. If you drop 12 thousand men on the shore, and they march inland, who pushes off and rows back? No one. The ships of the entire army are there, and only a handful could be pushed off the beach now that the manpower is elsewhere.
Obviously only after we obliterate this insult and gain some useful information from a captured shipsmaster or some dreck. Attempts to learn their language (it may be more beneficial to appoint a specific person in charge of learning all that they can about these raiders) would be very beneficial.
After raiding two years ago, they probably took home a fair number of kingdom slaves, perhaps those slaves have learnt their tongue, or they learnt ours, but they clearly had some intel on us, since they tank-rushed our capital. We're probably behind-the-curve on that part of the struggle.
See if we can take the settlement without much property damage or many civilian casualties. Hopefully, the settlement itself will be useful and the inhabitants can provide information.
Define civilian. There's no true civilian in Feroshire, as our levies prove, and there's no true civilian in those raider camps. At
Repton, where the Great Army wintered in 874, they even found women buried with weapons.