This is a
HUGE assumption that you make for literally all of your calculations. And it's an offhand comment buried in the middle of your post. It's also blatantly false on every single boss fight in Emerald Nightmare. At best, you're looking at physical, blockable damage accounting for 60% of your total damage taken on a fight. More realistically, blockable damage is 40-50% of a current boss fight in Emerald Nightmare.
Drake tank on Cenarius 55% blockable damage
Boss tank on Cenarius 60% blockable damage
Xavius tank 36% blockable damage
Ursoc tank 30-40% blockable damage
Those are considered the benchmark fights in Emerald Nightmare currently and none of them have "a vast majority of blockable damage." Nor, is the blockable damage the most threatening.
Haste and Mastery look like god-tier stats when you pretend that 100% of the incoming damage is affected by them fully. If you take the values you're getting for Haste and multiply it by .5-.6 and then take Mastery and multiply it by .65-.75 (due to it still affecting IP) you'll get much more realistic numbers. And that's where Versatility starts to make an impact.
Your post also pretends that you need anywhere close to 100% shield block uptime on boss fights. Again, this is completely not true.
68% shield block uptime on Xavius
96% blocked hits (same log, mouse over melee block to see percent blocked)
86% Shield Block uptime on Cenarius
94% blocked hits (same log)
And Clutch could have just played better. With ~80% shield block uptime he would have 100% blocked melee hits. This illusion that anywhere close to 100% shield block uptime is a good thing just needs to freaking stop (it's slightly better in 5 man content as you mention).