It’s because he got what he wanted and he knows that the WoL can’t do anything about it (and he REALLY hates the WoL so -10 to rationality). Hein even says that they have to go along with the plan even if he’s an evil shit because otherwise he risks leaving all the captured Domans stuck in imperial work camps till they die.

They can’t really plan anything because he holds all the cards at this point including important trump cards such as the fact Zenos isn’t dead and that the prisoner exchange is his idea. It’s one thing if it’s an angry Zenos zealot who’s rage is apparently aimed solely at the WoL; It’s a complete other thing if it’s Zenos who is puppeting them along.