A potential explanation I think is implicit in Ziz’s writing: the software for doing coordination within ourselves and externally is reused. External pressures can shape your software to be of a certain form; for instance, culture can write itself into people so that they find some ideas/patterns basically unthinkable.
So, one possibility is that fusion is indeed superior for self-coordination, but requires a software change that is difficult to make and can have significant costs to your ability to engage in treaties externally. Increased Mana allows you to offset some of the costs, but not all; some interactions are just pretty direct attempts to check that you’ve installed the appropriate mental malware.
“Complaining about your trade partners” at the level of making trade decisions is clearly absurd (a type error). “Complaining about your trade partners” at the level of calling them out, suggesting in an annoyed voice they behave differently, looking miffed, and otherwise attempting to impose costs on them (as object level actions inside of an ongoing trade/interaction which you both are agreeing to) is not. These are sometimes the mechanism via which things of value are traded or negotiations are made, and may be preferred by both parties to ceasing the interaction.