Averages reflect the vast bulk of game content. Boss fights are a comparable minority, and a smaller minority yet are ones that severely stress a tank's survival and healer resources. They're out there, but from a time perspective, something like 95%+ of one's interaction with the game falls into encounters where the statistical average is applicable and useful.
are the ones you need to optimize for. If they're not really a stress anyway, you'll be perfectly fine in under-optimized gear.
And really, this entire discussion boils down to relics, because there is precious reason otherwise not to have a single tanking gearset optimized for one's particular class (there's an easy-to-calculate mathematical "best", from which any deviation statistically means more damage taken). One is never better served by having more Defense for one particular fight, and more Shield for another. Other than ensuring sufficient HP (trivial at present), the math doesn't support a lot of customization.
False, it depends on the attacks used by the boss. If it's using M/R E/K attacks defense will have less value since an attack that gets trough the defense will be mitigated more by armor and shield, but if it's using M/R I/E attacks each defended attack will provide more damage reduction.
That's what's messing my calculations up at the moment - I want to optimize for Kephess, but I'm not sure what his balance of attack types are, so I'm going with the optimal values assuming only M/R E/K and then arbitrarily raising defense a bit when I mod my gear.