I'll see what I can answer:
+Magic Damage will affect both spell and weapon magic damage - so a bow with +100 magic damage enchant will add +100 magic damage to arrows fired, as well as to spell cast. A piece of jewellery with +magic damage will also boost weapon damage, not just spell damage (I just tested it).
What's more, dual wielding with two +magic damage weapons seems to 'cross charge' the weapons - each weapon gets BOTH magic damage bonuses. I just checked with my two magic damage maces (both similar in stats) and using one of them gave me +120 magic damage, using both of them boosted me to +620 magic damage! I don't know if dual-wielding +melee damage also 'cross charges' like this - I suspect it does, but I've only played as a caster.
So you will get bonuses to all special attacks that involve a weapon, such as rush attacks and exploding arrows, but I don't think this bonus will be any larger than if you just used the weapon normally. Spells that do not involve weapons or which do not cause magic damage (such as heal) don't seem to benefit from +magic damage though (but do benefit from +int bonuses).
As for your second question, the answer is basically 'yes'. For example, Magic Resistance is based on both your magic armour and your intelligence (and ranged on ranged armour and dexterity, etc.). I don't know the equation used off-hand, but it's not 1:1, e.g. one point of magic armour does not give you 1 point of magic resistance. Resistance is also capped (at around 75% if I remember right), after you reach the cap, further stat or armour bonuses will have no effect.
Hope this helps!
