Its all in the code! Open Source, ftw!
As of now, Magic Resistance doesn't do anything. Some discussion about it can be found here:
Link
Monster spells have a base damage and a range for random additional damage. The range of the later being set by Willpower. Right now Willpower influences roughly about 1/3rd of the damage done by most monster magic.
Magic Monstes have a set of spells and a variable for how ofthen they use these spells. They are not influenced by mana whatsoever.