Alright so let's test the mitigation at 1 scenario. Which is the heavy hitting 250.000 Damage tankbuster that targets both tanks. And let's assume both tanks have 100.000 Health
For this all tanks have to use all of their cooldowns to be able to survive the tankbuster. Which is Rampart + 30% Mitigation + Reprisal + 3rd button (Dark Mind/Thrill/Camo) + Personal (TBN/Shell/HoC/BW/Interv).
WAR = 20% + 30% + 10% + (10% + 10%) = 59.17% Mitigation
= 250.000 * 59.17% - 20.000 = 82.060 Damage
GNB = 20% + 30% + 10% + 10% + (15% + 15%) = 67.22% Mitigation
= 250.000 * 67.22% = 81.931 Damage
DRK = 20% + 30% + 10% + 20% + 10% = 63.71% Mitigation
= 250.000 * 63.71% - 25.000 = 65.720 Damage
PLD = 20% + 30% + 10% + ( 20% + 15% ) = 65.72% Mitigation
= 250.000 * 65.72% = 85.680 Damage
So those are damage taken individually if each tank are only using cooldown for themself. Now to test Intervention, let's take 3 tank combination, DRK+GNB, DRK+PLD, GNB + PLD. With PLD using Shell + Interv, and the co-tank in PLD pairing uses their personal on PLD to maximize the % mitigation on both tanks.
GNB & DRK
GNB = 20% + 30% + 10% + 10% + (15% + 15%) = 67.22%
= 250.000 * 67.22% = 81.931 Damage
DRK = 20% + 30% + 10% + 20% + 10% = 63.71%
= 250.000 * 63.71% - 25.000 = 65.720 Damage
Total Damage Taken = 147.651 Damage
PLD & DRK
DRK = 20% + 30% + 10% + 20% + 10% + (20% + 10%) = 73.87%
= 250.000 * 73.87% = 65.318 Damage
PLD = 20% + 30% + 10% + 20% + 15% = 65.72%
= 250.000 * 65.72% - 25.000 = 60.680 Damage
Total Damage Taken = 125.998 Damage
PLD & GNB
GNB = 20% + 30% + 10% + 10% + (20% + 10%) = 67.34%
= 250.000 * 67.34% = 81.648 Damage
PLD = 20% + 30% + 10% + (20% + 15%) + (15% + 15%) = 75.23%
= 250.000 * 73.78% = 62.903 Damage
Total Damage Taken = 144.551 Damage
So from the tests above, PLD is going to be operating at the same level of DRK in terms of mitigation. Even if the co-tank is un-cooperative and choose to your their own personal on themself, the total damage taken will just be a little bit higher due to inefficient mitigation % stacking.
Also what makes Intervention unique compared to all buttons is as you said it can be stored to have 2 stacks. Usually on Savage/Ultimate tankbusters would be spread 60-90 seconds aparts. With PLD gauge restoring 1 usage of Shelltron at a rate of 20 seconds, a PLD can easily stock up 2 charges while still using 1-2 Holy Shelltron in between tankbusters