Honestly, I don't recommend applying real-world reasoning skills to what they do in this story, ever. It's better if you think of it as a fantasy story adjusted for young readers, with heavy plot conveniences included by the author pretty much everywhere, and an intentional detachment from real-world logic.
If this is about the reparations between the two countries, then it puzzles me that one would see this as not being real world reasoning as opposed to a fantasy one.
Someone mentioned that as the perpetrator was representive of the assembly, that it goes to the highest levels and therefore is grounds for war. That doesn't make any sense in the real world. Imagine going to war because a minister or assembly man of a republic committed a crime in another country....an already disgraced one at that. To equate it to being equivalent to the actions of royalty of an absolute monarchy doesn't track in the real world.
Furthermore, as the people repeatedly note, no actual harm came to anyone on the Esfort side. Even if this was due to the efforts of Esfortians, the fact remains that there was no damage. Even further, the true mastermind was an Esfortian. While a mere member of nobility doesn't hold the same representative power, even a Marquis, and the actual grave crime was committed by Ilstin, this should be grounds for some compromise.
Judging by the fact that said assembly man's wealth was drained by the fine of 80 platinum coins, and that he was particularly well off, we can get a ball park estimate of somewhere in the range of 100,000 platinum for the country's GDP, give or take an order of magnitude, and assuming Renaissance era economics. If we take it exactly as 100k, then 10k even if over 10 years, is a very hefty reparation I think is quite valid given the avoidance of war and the circumstances of the people involved. Furthermore a territory was ceded, including a tax paying city that benefits from trade revenue, as well as priority on trade agreements given. Even if vague, these are at least as valuable/damaging, if not more so, than the monetary reparations.
I'm not saying this series doesn't have contrivances or fantasy reasoning, but the manga version does clean up a lot compared to what I heard/vaguely remember of the novel versions, and compared to other fantasy series I've read. It does a pretty good job of making the setting somewhat believable, for example explaining differences in local economy in different parts Esfort using geographic reasons in some previous chapters. I'm opposed to folks not thinking this through then attributing their objection on the excuse of realism. It reminds me of things like audiences panning some practical effects in movies as CG because they can't tell the difference.
If I seem quite anal about it, it's probably because this kind of stuff will become even more relevant in future chapters, just not with Ilstin. At least I think so, I can't quite remember.
Also LOL this king. Honestly he's a nice dad to have.