The problems here for everyone saying he should go in strong-arming it to force them to accept his terms are:
1) The information Kuina gave him should've told him that if the merchant guild didn't bite then the other guilds wouldn't. I don't know why he was acting surprised about that. He had days to read over what Kuina wrote down for him.
2) He had days to plan a method of attack for perhaps getting them to negotiate close to what he wanted. There was never any guarantee or never is a guarantee in a negotiation that you're going to get exactly what you want.
3) Why was he the only one in the negotiation? Why didn't he get Chloe to ask Kuina to attend if Kuina is aware of the importance.
4) If Kuina is aware of the importance, I get she wants to hold on to her grudge, but this would have obviously been a character development point for her. Yes, she wrote everything up, but only when he left the room. This shows that she's being a bit childish at this point about the grudge and it's not so much just about grief. She admits he's not the fault, but she admits she's holding him at fault just because she wants to so she can have someone to hold at fault because she thinks that will keep her friend's memory alive. The author should've had Chloe say something to her about this.