Remaining straws for Soo-won lovers to grasp:
i. We don't know whether he did indeed read the memoir. I was sure he had till last chapter, but now it seems the memoir went to the castle and never came back. At the very least, I think Soo-won didn't find the letter in the bookmark.
ii. Il was convinced, Yon-hi was convinced, Yona is convinced that Yu-hon had Kashi murdered, but the truth is that all the account is second- or third-hand. We'll have to wait for Hyoo-ri to admit it while engaging Hak in combat to clarify that, but I'm convinced, too.
iii. Whether Soo-won is the one ordering the mobilisation against the Kai Empire. That's a done deal for me. He's too much a Machiavellian schemer (in the bad sense of the expression) to not be the one in control of his own government. And he even said during the Xing campaign that settling that issue would make Kouka strong enough to face Kai.
I guess Soo-won lovers will have to be confronted with the "unstoppable scorching wave of bloodshed" (chapter 12; apparently Kin province wasn't enough) to be convinced. I personally can't wait.