I feel as though there isn't much of a diplomat game in existence right now, in regards to any worries. My anecdotal evidence as Ruler and ambassador/diplomat over several characters seems to hint towards this conclusion. There's next to no use for them as most diplomatic interactions occur over Ruler channels and impersonal mediums (letters.) Heck, I've seen this point in-game multiple times as well.
I like this idea a lot, in contrast, as it has the potential to promote personal/physical interaction among diplomats and revives some kind of importance to them outside of signing treaties and slandering/praising other nations to NPC's. This could offer backroom politics, betrayals, trading of information, and anything included in traditional noble scheming.
However, I don't believe government members should be invited, unless they're from the hosting realm or are diplomats themselves. If they are invited, then you once again deter the whole concept of including diplomats, as the Ruler replaces his or her importance immediately. Government members should remain in their nation as they have obligations to do so, unless on military campaign or RARE state visits, while the diplomats do the field work in foreign nations. That's how it was, how it should be, and how it could be with this request.
+1 to the original post.
The other way to massively increase the use and power of diplomats/ambassadors would be to just remove the ruler channel.
