I shouldn't laugh, but the way you described it, I finally see it as some Jon Woo shyt
he didn't have the presence of mind to swing back/protect himself, but he kept his composure enough to masterfully take out his gun, cock it, and shoot
and i havent even addressed how plausible it is for a man having his head slammed on concrete to be able to loudly and clearly scream for help so long, or if a man with a loaded gun is likely to ever scream for help at all.
i suspect that trayvon knew he couldnt get the gun, so he was trying to keep zimmerman from pointing it at him and was screaming for somebody to get him out of that situation.
i think that for the duration of the screaming, trayvon is either on top, punching him with one hand, and trying to keep zimmerman from pointing the gun with the other hand, or they move to a standing position while struggling over the gun.
the standing position is pretty plausible when you consider that the witnesses to the fighting only see parts of the fight, not the entire fight, not the screaming part, and not the shooting part. so the fight could have gone from the ground back to standing. it also makes sense that so little dna was exchanged between them. trayvons bleeding heart doesnt get any blood on zimmerman even thought he should be directly on top. when shot, his body should drop onto zimmerman, and if zimmerman is as short and weak as everybody keeps insisting, he should have struggled to get trayvon off of him. instead, the witnesses see them separated, and trayvons body on the ground with his hands folded underneath him
so however this scenario works out, whether fully on the ground or having transtioned back onto their feet, i think the fight was eventually over the gun (neighbors heard arguing before the fighting. this is a chance for zimmerman to pull a gun out) which i think causes trayvon to fear for his life, and zimmerman to be hit in the face which in turn causes him to shoot