Even though this idea heavily encroaches on Scholar, I feel like Plenary Indulgence would have been much more useful as a barrier effect instead of a heal.
  1. Ally takes damage
  2. WHM heals ally and gets Confession proc
  3. WHM uses PI and grants a minor barrier effect to mitigate a small amount of future damage
Now PI can be used freely when there are stacks to burn without having to wait for damage to be taken to use it. It could also have Paladin's Divine Veil barrier style, where "HP recovery via healing magic" dispels the barrier. Then, to make sure WHM isn't overflowing with barrier effects, turn Divine Benison into damage reduction for 10-15s (like PvP Protect) instead of a barrier.

TL;DR New Plenary Indulgence
Applies a protective barrier on all nearby party members that prevents damage up to a portion of the target's maximum HP. Potency varies with current Confession stack.
1 Confession: 6%
2 Confessions: 7%
3 Confessions: 8%
Barrier effect ends upon additional HP recovery via healing magic.
There is a 20% chance that party members are granted a Confession each time you heal their HP via Cure spells.
Confession Duration: 30s
Can only be cast when at least one party member has a Confession granted by you.