Yep, Armor is a multiplier of both maximum and healed health, Having double the armor would make your healing almost twice as effective.
If you are willing to go mid roll yes, Better full mesh but you could have leather gloves and helmet.
If you want to stay in Fast roll it is convenient to have a mixed cloth/leather or just leather.
Try with the armor you have if you see that the problem is survival then either you have to become very skilled at not getting hit or you move on to mesh.
Same, If you are interested, read this: The RNG Problem, Enchantment System & Gem System “TOGETHER” (Update) and if you agree, give your vote 
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I don’t think it’s any stronger, but it does get more focus.
If you like that weapon I think you might like this one too: Crooked Key
This is one I have used and I know for sure it is strong.
It’s a wand but it has a very nice moveset and the electric runes are very strong.
No.
You can only use the Radiant Ember on the rings, But they can no longer be sublimated.