One thing that's struck me on diplomacy from my Unification game (still setting up, that'll teach me for starting 10 Empires on one planet!)
I think the automatic change to diplomatic score might be a little extreme, as all my empires have embassies with each other, even the un-friendliest of them will be moving up the attitude tree towards mutual respect and admiration pretty quickly.
I don't really have a solution, unless you do something like add an attitude to diplomacy, so you can set how you interact with other nations, ie; you can set your diplomatic attitude to; Friendly, build co-operation; to increase your diplomatic relations, or to Watchful, distrustful; wont make relations worse, but won't improve them either, or Insulting to slowly lower your diplomacy, trying to provoke them to declaring war.
and actually maybe a penalty to all your other diplomatic relations when you do declare war on someone...