I'm not talking about tankbusters at all. Mitigating tankbusters is a fundamental pass-fail check on a tank. I'm assuming a basic level of tanking experience where you've mapped out not only every tankbuster, but also every cleave and AoE, and are starting to look at ways of spending as much of the fight safely out of stance as possible. In situations like these, you use your cooldown rotation to soften damage spikes from tanking out of stance.

While proc-based defensive abilities like parry and block are "unreliable" for tankbusters, they really start to shine when you look at them across a large number of hits (i.e. autos). Remember how I said that about 40-70% of the damage in a given fight is auto-attacks? Often this represents upwards of 600k damage over the course of a ten-minute fight. Having a shield may not be of benefit on a given tankbuster, but plays a significant role in controlling the overall rate of incoming damage, allowing you to tank out of stance.

RoH is a dps loss. But it's a bigger dps loss to use RA in ShO than it is to use RoH in SwO. It's better to make a small concession than a big one. If RoH keeps you out of tank stance, use it.

DM is not reserved for tankbusters. You can mitigate any tankbuster set comfortably using level 50 PLD cooldowns or their equivalent. You have a lot of bonus cooldowns at your disposal. A cooldown should never go unused for longer than its recast time. If it does, you should burn it to allow yourself to drop stance. DADM is a bit like IB on WAR. It's convenient if you somehow mess up your cooldown rotation, but it's also a dps loss if you end up having to use it. Regular DM is extremely versatile and has some use even in predominantly physical fights, where you can always burn it on an AoE heavy segment.

The key difference between HG and the other cooldowns is that it prevents damage. This isn't just a clutch save issue. If damage is a requirement for a stack or debuff mechanic to activate, you break the mechanic. It's one of the most powerful abilities in the game. You generally will use it twice per fight, as most progression fights are longer than seven minutes in length, and you don't hold a cooldown for longer than its recast timer.

Tank swaps result in dps losses. You can minimise this with good execution, but it's still a loss. This may be more a consequence of the fact that DRK/WAR tends to be less hung up on the "I'm MT" mindset, but my personal preference is to swap at natural points in the fight rather than swapping back simply to artificially preserve MT/OT designations. Cover is a niche skill which gives your raid an alternate way of working around this issue. You don't have to use it if you don't want to, but the option is there.