Paladins have terrible snap aggro, its just how it is at the moment. Changing from SoI won't help in the slightest. Try and time
Light's Hammer or Consecration to help with aoe threat, or holy prism or w/e you're using. You have hunters and rouges in your team so call for MD or something. I have 588ilvl and still sometimes get out threat between waves when my equally geared dps press buttons. If you have no vengeance then you're not going to hold shit xD
Btw, Glyph of Hand of Sacrifice is never really needed for a prot paladin. No one is going to take that much damage that sac will even come close to harming you, and if you have put it on a tank who is taking damages, chances are that means you're not. Sac will only transfer 30% damage at a time. Your target would need to take his max hp in damage 3-4 times over for it to do its 100% max hp cap
Edit: Except maybe if your cotank is solo soaking Malk, he would probably take his max hp 3-4 times over in 12 seconds then