Can any devs confirm or deny the Effect: Guardian's Boon being restricted to 1hb attacks only? I would think the effect would say that in the description if it was actually intended.
It is 1HB only, you can confirm it yourself in the spell file. Whether that was intended or not is another matter. You'll want to get the Gladiator's Plate Chestguard of War instead for the Guardian's Bravery effect, which affects all skills.