Thanks for the reply.
1.) That seems open to exploitation by the human player -> get open borders with a civ, set your units to an attacking position within those borders, ask around if anyone's willing to pay you to go to war with that civ or something like that...
2.) That seems counter-intuitive and I don't see what purpose it serves within the game. I mean, why I am being punished for solely defending my country without losing any troops? Why should my population be unhappy over killing the invaders? I'd understand war weariness if the war started to last too long with me winning and not accepting a truce or war weariness from losing my own units or from killing/damaging units in foreign countries (i.e. the public wouldn't like me being an aggressor), but not from this. Or at least I'd wish it were significantly lower than the "normal" war weariness.