I agree. This is no spellcasting we have here. It is martial arts.Personally, I think you're making bending failure a little complicated. If you want to make it very simple and straight forward so benders don't wear armor without taking appropriate feats, just say the armor's ACP is doubled and applied to the actual bending check (ala Swim checks in Full Plate). Simple, straightforward, and easy to understand, and create a chain of feats that a bender could snap up and use to improve armored bending.
Like, Armored Bender (light), then Improved Armored Bender (medium), and assuming you wanna go that far Greater Armored Bender (heavy). Those feats eliminate bending failure in that grade of armor.