If George Zimmerman really was injured, there's bound to be photos of it entered into evidence, which we aren't going to see until there's a trial. It's very likely he was cleaned up before being carted back to the PD if he was hurt. The video just shows he didn't have any wounds serious enough to get picked up on crappy surveillance video.
There's waaaaaaaaaaaaaaaaaaaaaay too many people involved, from the neighborhood witnesses after the fact to the police on the scene to the paramedics involved for this to be some kind of lie and cover-up re: injuries. More likely Zimmerman was lightly injured/cut/whatever and was treated at the scene.
re: the shot in the back stuff - I think if that had happened we'd have known about it already. The family's lawyer would have made sure of that.