There is a theoretical point at which Agi becomes
more avoidance per point than Dodge due to it scaling with Kings but that would require far more Agi than is ever reasonable in a tank set. Agi will simply never provide more Dodge than Dodge in current content. However, the reason many Paladins do choose to go Agi/Stam over Dodge/Stam is that the diminishing returns mean that the difference between the two becomes negligable - you might be looking at a loss of 0.08% Dodge or something, so the fringe benefits of Agi become more attractive. Also, with the slight extra bit of armor, the net effect on your TTL is more or less exactly the same assuming normal physical damage.
In terms of Armor, Agi offers two points of Armor per point. While 20 Armor from a gem is hardly noteworthy in itself, it is still EH that you can't get from any other red-matching gem.
As for your second question, it absolutely would help to maybe look at some of your enchants but don't go overboard. I see a lot of people saying "Stam goes in every hole" but they're effectively trading 10 Defense Rating for 3 Stamina, which doesn't seem like a good tradeoff to me. While TotGC
massively favours EH, it's not like there aren't elements of the fights that do warrant having some avoidance. For gems, I match only +9 Stam or higher with Agi/Stam or Def/Stam.
In terms of enchants, if I were you I'd switch the Chest back to Defense and keep Defense on the Shield but maybe try Mighty Armor on your Cloak. The PvP enchant is also certainly worthwhile if you can make it fit alongside the Quel'serrar and Mark of the Relentless. While the 15 Resilliance does mean you can dip to 536 Defense and still be uncrittable, I'd personally try to stick at 540 or higher to ensure that your Ardent Defender proc is returning its maximum health.
Hope that helps
