Would PLD be viable if its mitigation was as superior to DRK as its DPS is inferior?

[Dhex]

So if I understand what you wrote, the issue is not so much DRK being better than PLD but WAR being much better than both DRK and PLD?

Precisely; Paladin and Dark Knight both rely on being the primary target for their benefits to count; both lack TP utilities, both tool-sets require that they be attacked - Reprisal and Block are useless when you're not being attacked. Warrior's tools allow it to buff slashing damage, debuff damage, and it has an actual damage stance that puts out more damage while off-tanking. Dark Knight is close but it's also benefiting from content that plays to it's strengths. Paladin does the least damage of the three - and brings no meaningful debuffs when tank-buster & raid-wide damage is primarily magic.

Paladin and Dark Knight need to be balanced in comparison to Warrior such that WAR/DRK or DRK/PLD or PLD/WAR all work equally effectively with any of the three tanks in both the off & primary tanking slots respectively.

Edit:
Why though Dhex!? Because otherwise Dark Knight and Paladin become "flavor of content" tanks that come and go depending on what Square implements next. Warrior is guaranteed a spot because of it's damage and utility.