I'd be very surprised if George gets away with nothing in the end. The world these days, it's enough to make some passionate individuals go and bring forth the death threats they've been giving him.
On the Top vs Bottom issue, bullet entry/exit tests, ballistics, whatever GSR and residual tests should hopefully be able to determine the proper information on whether George or Trayvon was on top when the shot occurred.
I have a feeling that the eyewitnesses only accounted for portions of the fight and it is entirely possible the two changed positions rolling on the grass. There are other events that could have occurred during that small window of time, and if George actually shot Trayvon while he was on the ground, it only can look worse for his trial.