D&D 5E Player feedback: Deserved Easy Win or a Satisfying Win?

[Blue]

No, the PCs in the example have literally been publicly agitating against the Duke, running a vocal propaganda campaign (among many many other moves against him).

Well, not publicly. They have spread rumors in private, and have taken care in doing so to both hide who they are and even that these are specifically targeting the duke - giving them much harder DCs to meet. And they have.

If you're curious that would likely be the crime of sedition, and likely carries a death sentence.

Oh, they already are under a death sentence for threatening to kill the rightful head of the Imperium, the Emperor-Regent. Whom legally got himself install above the Child-Empress they are sworn to. Even though the party calls him the Ursurper, he his legal, and this duke is one of his strongest supporters.

A bunch of mid level PCs (these guys sound like they're roughly in the teens) dont just walk into town and no-one notices either. They're literally capable of taking down small armies at this point; they can usually be expected to be met at the entrance to most towns, tabs to be kept on them, and for the movers and shakers in that town to get them on their side.

They are 8th level. An oinoloth is only CR 12, if they were in their teens this would be a cakewalk. And they didn't walk into town, they snuck into town, having used disguises, forged identity papers, and false names since they passed the border over a week ago. Much of the time they were also under the effects of an item that prevented scrying. None of that was detailed in the post because it wasn't really relevant until the point came up assuming they were doing things overtly, which is not true at all.

Unless they've been extremely covert, working through intermediaries etc, the Duke is going to know what they're up to. Not their exact plans of course (unless he's scrying on them, or spying on them, which I would be if I were him), but all but certainly word has reached his ear (at a bare minimum) that:

1) There are extremely dangerous PCs in town,
2) They have been actively spreading propaganda against him, in an effort to undermine him.

That's my only point here. If we assume that's true, then the Duke does what the Duke would do with that information. The PCs actions still matter (and he's still weakened) though.

They have been extremely covert. Most of their prep has taken place outside the capital. There is no indication that a powerful adventuring band is even in the duchy - the one combat thing they have done was eliminating to a man a zombie plague outbreak. There was a single civilized survivor, a halfling farm-hand who (a) was extremely grateful for his life, and (b) there is a halfling folk-hero in the group he wouldn't betray. While in town they kept a low profile. When I said they did their prep very well, I was not kidding.