I understand what you're saying, but Gathan isn't at that point yet. He's in limbo. All he's got going right now is hockey, and that's clouded by his conflict with Matt.
I think that your comment (underlined above) is closest to the truth. I don't think JJ was ever picked on, at least not extensively, what with Will and Darius as his brothers, but living in their shadow couldn't have been easy. I'm thinking "Jan" of the "Brady Bunch." I think that's why he's so obsessive about skating; this is finally something that he does better than anyone in his family, and probably better than most people on the planet.
When I wrote the scene where JJ turned those guys in, I was thinking in terms of him lashing out not just at them, but at Will, and stating that he was going to take responsibility for himself. A cupcake explosion: interesting. Now that could be one fun scene to write.