It's very possible that Yotsuyu wasn't the only courtesan working for the Garleans, but I still say that unless you've been in that kind of shit, you can't really judge someone. You really need to be in a position of utter hopelessness to understand. At that point every choice would be better off that your reality even if that means sending your country to hell, a country that allowed your suffering.
It's not something you can really realize when you are not in that position, she said it at the end, that her countrymen decides to turn around to ignore what's rotten to believe that the wrong ins't there. Hien as a leader has to fix this one way or the other because he has witnessed what the old ways lead to. Trye the Garleans were the abilitator to this, but let's say there was no invasion, this would have been the custom forevermore. He knows now and can either pretend nothing happened or take actions, but I guess we won't see it.