I may be missing something obvious here- but is there a way to have private, diplomatic negotiations, or do they have to be here in the public view?

Hanibel I'd like to negotiate...

Railhound - NO

Normally as soon as the Horde started to attacked me I would have tried to Treaty with you. But as been pointed out in other posts the game mechanics this time around are different. 
So I have been experimenting with ool strategies (which are not working as well) and developing new ones for the next map.