The -60 attitude modifier is an old one, so having the Cossacks doesn't matter when it comes to calling the AI in. Cossacks just allows you to actually set your stance on the AI nations, instead of being an inscrutable question mark.
Truces double dip on the reasons to not war, even though the -1000 is more than enough; they also get -250 for would destablise the nation. Military access I don't believe gives any modifier, but a -40 attitude towards enemy would be because the ally feels threatened by the enemy.
Good point, though then the system is liable to put you on Domineering or leave you on Neutral. Could even leave you on Unknown. Or it has you on Friendly except for that one month where it switches you to Neutral, which your allies immediately notice and start that war
Unless it's changed, it's -50 for each point of stability they'd lose. Since breaking a RM is normally -3 stability, it's generally -150. Not sure if the ally having full Diplomatic Ideas cuts it down to -50 or not. Like Gartak pointed out, there's no truce, as that would have a -1000 modifier labelled as such.