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Keep both rings and trinkets. The Seal of Darkshire is too low in terms of ilvl to outweigh the loss of haste from your other ring. I also wouldn't take the Raven Eidolon since its stats aren't really great (no haste) and i wouldn't look for stam-trinkets at all. As Prot-Pallys we have pretty much the lowest health pool of the tank classes, but you won't face problems just because of that. The on-use effect isn't that great either. Look for trinkets with haste as a stat or on procc, like Unstable Arcanocrystal, Chains of the Valorous, Chrono Shard, ...

If you're new to pala tanking stacking versatility is the easier way to go. As you get more experience you will want to go for more haste and mastery (value increases with more haste). With more haste you can play in a more active way, more self-heal through LotP, more active mitigation with SotR and so on. I'm saying experienced as you have to manage your active mitigation better when heading for haste.

I don't think it will work, the tooltip says that it will only hit one target. I have the legendary legs though, and I will try as i get home in a few hours and edit my post then. /edit tested it - doesn't work, just hits one target