It's pretty clear we have two fundamentally different ideas of what a government is, so I'm not really expecting to sway you at this point.
What you're describing is functionally a veiled version of anarchy - something you could certainly make a case for, but not really the system we're talking about here.
We pay the salaries of diplomats, of course. They work for us. But hiding information is really part of the job description. I'm saying that diplomacy just cannot occur if you can't go "back home" and talk to your colleagues candidly about other countries. Yes, behind their backs, and yes, in disparaging terms, often enough.
I know I keep going back to this, but seriously, please describe to me how you envision global diplomacy where private conversation is banned. I just cannot see it happening.
Low-level diplomats, who are hugely important in the day-to-day world of foreign relations, don't have the authority to make decisions past a certain point, of course. In your scenario, they would need to PUBLICLY go to their superiors.
Can you imagine how many controversial, potentially catastrophic, stupid ideas need to get knocked down by higher-ups? Can you imagine how much worse those ideas would be if we found out about them during Phase Zero? How many times would we see missile-lobbing and embargoes between countries because an idealistic 23-year old had a stupid idea while brainstorming? It would be hell.