1. No. Rage has very little to do with it. It's all about whether your raid needs more DPS or more survivability for the content you're doing. Stam/Mastery for survivabilty Parry for raw damage. raid bosses for the most part do not challenge tank ehp until you fuck up the mechanics
2. there's a cheap and dirty macro in my guide but If you want real maths Theck's got a couple of very mathy Blog posts worth reading
In a stam/mastery build you do want to balance your avoidance, but for the msot part it pretty much boils down to reforging any bit you haven't already reforged to meet the hit/exp caps or maximise mastery to dodge since you'll generally get enough str from gear to push you heavily into parry's DR. In an Avoidance build you still don't really care about balancing becuase Parry thanks to GoHtL (and a little bit to parry-haste) menas that Parry is much much better for your DPS than dodge is.
3. 24 seconds followed by 3 seconds of down time. it only works out to be 12 seconds of down time if you need to setup another 24 seconds of Sblock coverage. The value of Mastery as block rating is pathetic, but it doesn't matter becuase unless you massively under gear the content no boss can two shot you (unless you fuck up the mechanics and take more stacks of tank debuff than you should), and with a 3 second gap between shield blocks they kinda need to do that.
The rage gains are practically identical in tier 15 gear. they might be identical in lower gear, i haven't simmed anything below 522 ilvl with 5.4 mode but practically any setup should give you more than enough rage to keep Sblock on CD. Back in 5.0 in 463 gear with every gem and reforge and a trinket to cap out hit and expertise you could generate enough rage to keep sblock on CD. If you're not generating enough rage to keep Sblock on CD you're doing something very very wrong. Rage isn't an issue.
You need to stop comparing pre-dr block for pre-dr parry. it's meaningless, the combat table doens't work like that anymore. As a warrior you do not care about your block chance any more than you care about your in combat mana regen. on fast taunt swap fights you really do have very little opportunity to be bursted down since you'll generally be taking the boss for 30s out of each minute so you can easily cover yourself for with SBlock for 24 seconds and demo shout for 10, without even attempting to use Sbar to plug the wholes in SBlock uptime. On fights where you have the boss for longer stints, you generally get to throw disarms in or are fighting packs of adds which don't really hit very hard, or are having to deal with non combat table frontal cones.
and their are the legendary bits.