List of WAR buffs 2.1

Not sure where you have found the the full time 15% mitigation from? There is a 120s Vengeance (with a 15 second bonus uptime), that provides both a 50 damage counterattack and a proposed 30% damage reduction. While the uptime is the same as say, Rampart (90s skill, 20% reduction, 15s uptime), the 2 minute timer means it will be utilized much less (4 ramparts to every 3 vengeance.) We will also not receive Sentinel (40%dmg reduction, 180s, 10s uptime), which is a CD that can be combined with Rampart, and provides a much more significant benefit.

Innerbeast also provides a 20% dmg reduction but only after achieving a full stack of wrath and only for 6 seconds. Assuming a warrior does not receive wrath from Infuriate, Berserk, or Vengeance, they will need to use 8 attacks off the Heavy Swing combo branch plus Inner Beast (a total of 9 GCD over about 22.5s) to receive this benefit.

Assuming all of the previous effects of damage reduction are additive; a Paladin with Shield Oath, Rampart, and Sentinel active will receive a 80% damage reduction while a Warrior with Inner Beast and Vengeance will receive 50% damage reduction with an additional 25% HP.

On Holmgang: I'm not sure how the benefits of this ability can be considered comparable to PLD's Hallowed Grounds. A Paladin who uses this ability can activate it at any time, has full mobility, receives no damage, and cannot be killed. A Warrior who uses Holmgang must first target an enemy, loses all mobility (cannot be knocked back either), may bind the enemy (many enemies are immune to bind), and receives full damage up to 1 HP.

link_dead within the reddit thread makes a point regarding how Hallowed Ground also benefits healers. Its also important to note that the 6 seconds provided from Holmgang really only gives healers two GCD at most to heal the Warrior up from 1 HP.