[Micah]

Other sources of MR stack, with the exception of in-battle spells.

As a general rule you can only get one instance of a buff cast on you per battle, so Iron Will, Resist Magic, and Antimagic will not work together, you'll only get a single +4 bonus to your MR. Similarly, stuff like Gaea's Blessing won't stack with other resist spells (The various wards and warriors spells)

I'm not sure how the bless bonus interacts with other bonuses though.