Please point out some examples of where actions or thoughts have become dysfunctional or irrational.
From what I can see from the Abwehran / Iroma interaction, it's just commentary more than actual Groupthink. They are the two oldest delegates in the conference, which is bound to lead to them grouping together to make such small talk. Yes, they have a better cohesiveness than the others due to having longer lasting trade agreements and diplomatic relations than they've had with the others. They may even have a bit of Unquestioned belief in their own morality. However, I don't see any other symptoms of Groupthink in their discussion. No illusions of invulnerability, not too much closed-mindedness (though enough due to being long-lived individuals), and the only stereotyping I can find is on levelheadedness. There's no illusion of unanimity (since they have friendly relations) or signs of self-censorship.
The Nepleslian and Lorath aren't dysfunctional in the slightest, even if they are pouncing hard on Yamatai. Their actions might be construed as irrational, but they're mostly playing hard-ball in order to get what they want (which happens in both politics and diplomacy). In essence, they are acting in a Groupthink-esc manner in order to get their own goals accomplished, which often can happen in gatherings such as this.
Yamataians and HSC would be considered close allies (from what I've read), which means Groupthink is quite a possibility --- if the HSC hadn't left the conference over the possibility that their treaties with the YSE are going to suffer over the new treaty the Nepleslians and Lorath forced upon the Yamataians. The only thing I could accuse the HSC delegation is of tactful disagreement towards Yamatai's signing of the treaty than the illusion of unanimity. There's strain in that relationship and no actual symptoms of Groupthink at all.
Overall, I believe you're jumping the gun in your conclusion.