Paladin's don't "double dip" block directly reduces the effectiveness of parry. Using your 30% block and 20% parry the paladin would actually have 14% parry. With Bulwark up that same paladin would have 90% block and 2% parry. You can't parry an attack you blocked, it is still solid mitigation, but not double dipping.

Hundred is also correct. Taking into account up-time and equal levels of block rate/strength and parry rate/strength for both paladin and dark knight, keeping both Bulwark and Dark Art + Dark Dance on CD the mitigation is identical. Meaning that Dark Arts + Dark Dance makes up for the lack of shield, or at least that's what it seems to be designed for. There will be discrepancies because some attacks can't be evaded. But block/parry/evade are more useful for auto attack mitigation than controlled burst.

Convalescence, Foresight, Bulwark, Stoneskin, Awareness, and Bloodbath. These cool-downs you listed the only one that is a little useful for defending against a tank buster is Foresight, and even then it's only a ~5% physical reduction. Convalescence just helps healers catch up but doesn't mitigate anything. Bulwark, even if it's 90% block there is a chance you take the full hit. Stoneskin can be cast by any healer, and with Clemency (can't be interrupted) now it seems like a giant waste of MP to cast something that any healer can do, also you can't block or parry while casting which is fun. Awareness only applies to crits and the only boss thus far that could crit was Shiva and it wasn't a tank buster. Bloodbath is just like Convalescence, just helps repair the damage not stop it.

Tank busters have always been more about the heal/debuff coordination than the tank. Cool downs are great and helpful, but unless you are popping Hallowed Ground, it will hit the tank hard no matter the cool down and the healers have to react. T5/T9/T13 if it's just the tank using Rampart or Inner Beast the buster is going to hit hard and be more difficult to deal with, but coordinate an Adlo, WHM Stoneskin, and Virus it becomes far more manageable.