About #5, I don't believe a whole lot of research has been done so far, especially since the game is in a high state of flux at the moment with things being changed around. However, one recent change is that each discipline has two stats that determine the damage of their automatic melee (And archer's Light Shot ability), and there was some juggling around in what stats do as well. You can read about these changes in the Patch 1.20 patch notes.

Now, that said... in that exact example you have up there, I'd say the +20 defense, based on looking at the materia available. Both defense and vit materia cap out at 16-20 for tier IV, so assuming SE actually did their job in balancing (And it's entirely possible they didn't), then 20 defense should be about on par with 20 vitality, most likely being more purely defensive as vitality also does other things such as max HP and marauder damage. Another reason to go with the 20 defense over the 4 vit is that you're gonna have an assload of the stats you need when you hit 50, 4 VIT is gonna be a drop in the bucket and very likely won't amount to diddly squat. Hell, the 20 defense is probably a drop in the bucket too. In fact, there's actually videos up of people taking down high level mobs wearing nothing but holiday event gear. Outside of the stats on your weapon for DoW (As that's where basically all your attack and accuracy comes from), gear doesn't contribute much. It's something a number of people are annoyed about.