@sgathach Well, it's a tricky issue.
In principle, Kai has been hiding behind the excuse that the slave trade and the nadai smuggling were criminal initiatives of private Kai citizens, without knowledge of the Kai authorities. As such, they're not acts of war because not sanctioned by the government of Southern Kai. So Kouka cannot state that invading Kin Province is an acceptable quid pro quo, because war is an affair between countries. As I said before, it would be as if the US invaded Mexico because of drug cartels.
What Yona did was get the emissaries to let slip that they are distressed by the possibility that Kai merchants and aristocrats imprisoned in Kouka might provide her with a report on their activities. The way I interpret this part is that the emissaries are afraid the prisoners will reveal their actions
were sanctioned by Kai's government, thus proving that Kai was trying to undermine Kouka by proxy.
That is an act of war and would provide Kouka with an excuse to invade Kai.
Personally, I think Kusanagi Mizuho is trying to throw a red herring and make us support Soo-won's plan to invade Kai. I won't rise to that bait. The evidence that Kai was engaged in foul play has only been unearthed now, by Yona. In principle, without hindsight, the emissaries' position is actually correct: Kouka
is the aggressor. There had been no suggestion so far that Soo-won bothered with finding Kai's government responsible for slave trade or nadai. He invaded Kin on spurious pretexts and is planning to start a major war because of his father complex.
I doubt Yona will support such a course of action, either. Even less so because she didn't support Kouka's invasion of Kin. But now she has incredible leverage over Kai's emissaries, and has the upper hand starting the conference. However, the emissary sitting at the head of the table opposite Yona - the guy at the middle of page 22 who hasn't spoken yet, is probably the main antagonist this time.