??? Did you even read the chapter? What you call "representative" is nothing more than a messenger; go to negotiation table, discuss terms, return to leader, report and receive instructions, go back to negotiation table, and repeat until conclusion is reached.
That's clearly not what happened here. Zenjirou was the one conducting the negotiation, using his own judgement. Aura even said so herself, that Zenjirou's decision didn't align exactly with what she would have done. That means that she wasn't the one directing the talks; Zenjirou was. A true representative needs the authority and trust from the one they are representing to act on their behalf. Otherwise, if their role simply was to parrot what they've been told to say, listen to the other party and go back, there would no need for an important person to play said role. A relatively trusted messenger would have been enough.
In any case, this manga doesn't really showcase well how politics and diplomacy work, since even acting as a proxy grants you a level of recognition for your achievements, however minor they may be. This idea that a representative gets absolutely no credit for their work is just silly. The world just doesn't work that way.
And you are overestimating what nobles can do. Zenjirou cannot become a puppet leader on behalf of the nobility if he doesn't let himself become one. He simply has to refuse to act in a way that benefits them and that's the end of it.
Also, the fact that Ines is not prepared to assist Zenjirou in his duties as a King Consort is a major oversight. She is his primary attendant and advisor when acting abroad, and has been (presumably) trained to support him in whatever he needs; that should include diplomacy.