Are my mitigation numbers off ?

[Reynhart]

Currently PLD is just about 10% under the other tanks. To make up for that you would need a vulnerability up debuff that adds about 2-3% to total party DPS.

Isn't that the same with slashing debuff ? WAR increases damage for NIN and other tanks, included itself. Of course, it doesn't increase the damage for the whole party, but you can have it applied 100% of the time. Adjusting the uptime of the vulnerability debuff is an easy to prevent too much DPS increase...and of course, not stacking it with Trick Attack.

As for the tankiness part, we know that that is a problem because of how 2.0 worked out.

2.0 was a little extreme on that. It wasn't that PLD had better mitigation than WAR, but that WAR had barely enough mitigation for the content.
Personnaly, I think they overdid it a little between :
- not any mitigation and healing boost tied to wrath
and
- constant healing boost to Defiance, mitigation in Inner Beast, mitigation in Storm's Path, mitigation in Vengeance and "ultimate survival" in Holmgang

Again, the extra mitigation is not inherently useless. Tanks survive because healers cure them. More mitigation requires less healing, thus healers could do something else...like DPSing.
The problem, in the current state of the game, is that the tank with the worst mitigation (Whatever it is) already allows healers to do lot of DPS, so, whatever additionnal mitigation PLD offers, it's not significant...whereas its personal lesser DPS is.

the uniqueness of the class should be from mechanics or unique, moderate-power utility skills (like Defiance or Cover).

Which is already a problem, since every tank use the same mechanic to survive. It could have been much more interesting to have one tank that survives through mitigation, one through self-healing, one though leeching, etc...
If they were, they could have the exact same mitigation and the exact same DPS, because each player would pick one depending on the gameplay. Exactly like how most DPS are mostly on par with each other, but with different mechanics.