I think one arm contributes to him being a stronger person, but would contribute better if we the rpers could see it happening. For instance the way you've written his CS (from an outside perspective) makes him sound like he developed the skills instantaneously. He takes out two men with one arm and has been rumored to have never been hit in training. Doesn't sound like the struggle a disabled character would go through to make himself better at all. Even when you add his faults it still sounds like his life was a piece of cake. The simplest solution IMO would be to edit those parts so we can feel the struggle of the character begin right from the CS. I think he thereafter could go on to work his way up to rank four.

EDIT: Ive just read what I think is your edited post which pretty much conforms exactly with what I am saying. I hope it hasnt tampered with your plans for him too much. Accepted!