I really hope he just walks out of negotiations. The kingdom has nothing he needs. The kingdom can't keep its representatives in line, meaning any guarantees with them are worthless. This is a complete loose/loose for him to continue negotiations. Or worse, a trope like a "duel" or "demonstrating your power". Works in fiction; not in reality.
I wish JP authors wouldn't have their "oh so humble" MC act like morons "for the plot", or fulfilling a template trope. Having MC walk out of negotiations would be equally as interesting a plot vehicle. Yet few of their authors thinks this through. Completely forgetting people obey authority only because of the threat of enforcement through violence. Maybe because target demographic is children, not adults? Who don't comprehend that, or who they try not to teach that?
What happens when a weaker kingdom attempts to insult a larger empire, or country with larger army? Or in this case, literal demispirit?
Unless there are political, economic, or social implications -- kingdoms kill the men, steal the women, and take the wealth. That's literally how every single human society, tribe, and group has worked since the dawn of time. The only thing stopping it in modern day is social and economic considerations. In medieval/tribal era, it was simple war.
I need to reread to see what MC's motivations are. I thought it was security and safety of his sisters -- and possibly a sense of responsibility for the kidnapped girls. Maybe I'm mistaken.
(Edit: Rereading Chapter 1.2 -- it seems MC/author has indeed lost the plot.)
If anyone doubts my commentary, just see
Chapter 2.2 Page 4. Spirit Queen is a proper Queen, and is in-universe for series tone.