Yep, all ranged attacks. I have a feeling that the 7th ed FAQ said it only worked against magic missiles with regards to spells, but that could be something else I'm thinking of, the 8th FAQ doesn't mention it. And moving this to rules questions.
Yeah, but if he is in a unit then only he will get the 2+ ward from the plaque, it would have to be a spell that targetted him specifically. The magic resistance would cover the unit like normal.
only if you aren't paranoid about cannons and failing your "Lo,S!" roll. otherwise, either MR3 if you run him with TG or divine plaque + MR3 if you run him solo. in that case you want to keep him away from lose combat.