1. I think you've missed the point. You would use shield block for most of the fight, and then drop a 60 rage Sbar to coincide with attacks that are unblockable or otherwise need more than just a block's worth of mitigation. You have to be fluid and choose the right ability to counter the current largest threat to your HP, this means you need to know how much attacks hit for, how much vengeance you currently have, how much rage you currently have (and thus how big an absorb your Sbar will be) and if the special is blockable. It's generally not a case of "for this encounter use exclusively use shield block/barrier", Generally it's shield block against white swings, shield barrier against dangerous specials. That said I would for heroics general go with a Hit=exp>stamina>mastery
2. I agree with Theck. For just spamming Sbar Mastery loses a lot of value. Most guides aren't built on the premise of spamming Sbar which is why they rate mastery and mastery hybrid gems highly.
as far as gemming hit/stams in yellows, yeah why not? if i'm not ignoring socket bonuses and gemming pure stam, it's 160 secondary stat. if I'm under the cap Hit is better than dodge, if I'm over the cap dodge is better than hit. it doesn't really matter where the stats come from as long as as i get 2550 hit rating and 5100 expertise from somewhere whilst not sacrificing stamina.
3. keep in mind that the only time I would be Sbar spamming would be if the boss did entirely magical damage (like Lei Shi)and my gemming will reflect that
yellows: stam/hit if I cant reach the hit cap or if the socket bonus is a net stam gain, otherwise Stam
Red: stam/exp if I cant reach the exp cap or if the socket bonus is a net stam gain, otherwise Stam
4. Ideally I would have both. See this is the is the major point I'm trying to make. It's not a case of "Shield Block, I choose you!" it's about spending rage properly. There's no reason why you can't watch the DBM timer, pool some rage before hand, get a shield block off less than 5 seconds before the bite, and then pop a 60 rage (or whatever) SBar as he's casting it.
On your armoury; I am really confused. Why do you have 10.74% hit? the cap is 7.5% anything above that has zero value, that's 1102 rating you could lose and suffer no ill affect. you could literally turn all your confounded gems into stam PVP resilience and it would have no effect on your PVE survivability. additionally, if this is an Sbar spam setup why are you reforging out of avoidance into mastery on your gloves, and why have the glyph of Heavy Repercussions?