Are my mitigation numbers off ?

[Reynhart]

Even your #s for rampart/sentinel are off since 2 forms of the same reduction are multiplicative not additive.

No, they're not. Rampart reduces 20% of the damage you would have taken without it. The fact that those damage are already reduced from the "initial" damage is irrelevant for the sake of comparing tanks, since, again, the three stances give you the same eHP.

It really didn't work half as well as mitigation. Like you needed enough gear to actually survive DS in order to get healed or use IB while a PLD could just survive with a cooldown+Stoneskin with substantially less gear and then power through the disease debuff (and apparently SE couldn't foresee that).

Even with average gear, I've never fell from full HP to under 20% HP (Without cooldown) with a single Death Sentence, so WAR could survive each hit by having their HP almost full beforehand and proper application of Stoneskin/Succor. And the proper healing boost and self-healing would have allowed you to replenish your health to full before the next DS.
In the current state of the game, it means that healers would be focused on healing you, so they would have less time to do some additionnal damage. With a PLD, they would have more room to cast some offensive spells.

I would also change Inner Beast so that any HP that "overheals" you go directly into your HP max, like some kind of barrier.