This is a GREAT read. I've got to admit, when I reached the last chapter here, I switched over to Fanfic and finished it there. Also, for the first few years of SPN, I was very resistant to AU's, finding everything I wanted in the Winchester Universe. However, over the years the show has reinvented itself and gone down several unpredicted paths on air. So, I no longer have a knee-jerk 'no AU' reaction and have consequently read some great AU stories.
I think timing is very important in AUs and you picked a perfect point to diverge from canon, before it really started, referencing remarks made on the show to fill out your younger versions of Sam, Dean, John and Bobby.
You also picked a wrenching way to go AU, giving Sam a permanent injury. Again, something I'm not usually in favor of-- I like the boys being externally intact, to hide all the inner turmoil, angst and damage and keep themselves in the hunting game.
Anyway, thanks for posting this on SPNville and I hope I've broken the review dam wide open for you;-)