I'm not sure I'd call being denied overly aggressive tariffs by taking their trade elsewhere abuse to the nation or an act of national aggression when it's being done by non-government organizations. Tariffs go both ways as well, and given the class issues made present in the Samurai and some Kugane quests it could be that there's plenty Hingashi who want their own goods going out into the world as much as there are traders from without trying to get their wares in and being unable to by such economic stifling though that's heavy inference.

They're not wrong to monitor and try to control the flow of imported goods, though, and I don't think that aspect of trade would change with the tariffs being dodged since they still have the sekiseigumi and the local magistrate tracking goods. They don't know the quantities foreign markets could send in, or what, if domestic goods get flooded out their economy can take massive hits.

Does bring up something interesting. Who would actually own Eureka? How far from Hingashi is it? Apparently far enough that they have yet to lay claim. As it's the Isle of Val does it still belong to Sharlayan? Since Sharlayans are backing the expedition to recover their own people and find out how it got transported does that legitimize their use as a port?