Tristan's depression and war weariness did not prevent him from continuing to act as a conduit between Ves and his backers.
When the man was not complaining about the death and destruction that his actions may have put into motion, he dutifully relayed the words of the weaker coalition partners to Ves and the Hexers.
Even though it might have been more convenient for the Carnegies and Vermeers to negotiate directly with the Hexers, it was impossible for this to happen.
The two sides were still enemies!
Not only that, it was incredibly taboo for the Fridaymen to openly plot for the downfall of one of its coalition partners!
By continuing to communicate through Tristan, the damage of exposure would be kept at a minimum. At most, a single Fridayman mech designer along with a few unimportant diplomats would take the fall for the scandal.