I would also be interested in this question, I can't find anything to prove or disprove it either. "Find Their Weakness!" would be another shout that would benefit from it, especially in combination with "Go For The Eyes!".
Yes, Minions can critical hit. The % is alot smaller than if you were the player or an assaassin. To tell they land a Critical hit there will be a big "Red splot" and a Louder noise when they hit. You will have to be like right next to your minion to see it. But out of all the summons, Fiends do the most critical hits. And the Rate i would belive would maybe be 1/ 20 hits. That is rounded to about 5% Chance. but thats based off of what i see as a MM.
There's an easy way to test it. Raise one mininion and then let it go attack a bunch of level 4's or so. If they can critical they should be hitting the same amount ever single time, because against a level 4 all of their hits should be criticals. I'd go test it but don't feel like switching my necro from SS to MM .