1. I'm pretty sure that the percentages are additive. I'm just making an assumption, although it's rather safe, since that's how other percentages are stacked in TES games. For example, the "Fortify Magicka" enchantments can stack four -25% to get -100%. There are numerous other examples as well.Note that there is a cap on Magicka Restistance at 85%; that's as high as you can go. According to the wiki, Bretons can actually reach this without even having enchanted armor: Racial Bonus (25%), Agent of Mara (15%), Magic Resistance 2/3 (alteration perk; 20%) and the Lord Stone (25%).2. Magic Resistance should work on dragon breath. However, you'll probably be able to get fire/ice resistance to higher percentages alone.3. Later in the game, you'll likely be nearly hitting the armor caps of 567 regardless of whether or not you invest in those perks (especially if you boost the stats like one of our guides mentions). It really just depends on whether or not you want to hit the cap faster at the expense of your perks. I wouldn't invest any into Heavy Armor for sure, just because that will hit the cap relatively soon anyways. Light Armor, maybe, but again, only if you don't have anything else you think your perks would be better spent on.4. The fortify enchantments could help if you don't want to spend your perks, and the solution wouldn't be permanent if you change your mind and want a different enchantment. It's hard for me to say since I'm not sure how much damage you're taking.