No, forensic evidence didn't prove that. In fact the evidence I read about suggested that the kid was running away when he was shot.
If I recall correctly, forensic evidence showed that the gun contacted his clothes and that it was approximately four inches from his chest. This is consistent with him being shot while on top of Zimmerman, with his clothes hanging down.
Honestly, this is ridiculous. Haf, the evidence as according to the trial shows that he fired in self defence. Now whether he was supposed to be there, or picked the fight with the kid is irrelevant. The USA has legal firearms. Therefore, if you're in trouble, shooting the guy is an inherently legal choice. So, he was never going to be convicted.
That's not how the law works. Using a weapon is not inherently legal if it's determined that it would be unreasonable to consider your life in danger at the time in which you use it. The defense showed that Martin inflicted some fairly serious injuries on Zimmerman and could be considered reasonable to use deadly force under the local laws. The jury happened to agree with this viewpoint and ruled not guilty. Laws about deadly force apply to all kinds of weapons as well as killing someone with your bare hands, not just guns, and the fact that he used a gun is mostly irrelevant. Any deadly force falls under the same rules.
It was Zimmerman's mistake to disregard the dispatcher's directive to continue following Martin, which allowed this to happen in the first place. He may have confronted Martin or Martin may have confronted him, but there's no evidence strong enough to prove either way in court. Who started the altercation isn't terribly important at this point though. If Martin did start it, it could be considered self-defense on his part (though the most prudent course of action would have been to call the police and to go home with all haste). If Zimmerman was the first to make it physical, it's still Martin's prerogative to defend himself. Either way he's not committed a crime. Zimmerman will have committed one (assault) if he started it, and not committed any violent crime otherwise (I don't know how Florida's laws handle stalking).
Once the fighting started both men are defending themselves. Martin is defending himself from someone who was following him and could possibly have the intent to hurt him, and Zimmerman is defending himself because it's quite clear that he was taking severe blows to the head. Deadly force in retaliation would have been reasonable in the eyes of the law from either side. It's sad that it got to such a point, but that's what the laws of the state dictate. Whether or not those laws should stand as they are is another debate, and something that will be examined closely now I'm sure, but the laws in force at the time allow for this verdict.
However, I don't think it can be ignored that Zimmerman hunted Martin down and eventually took his life. Regardless of how he felt (and self-defense hinges on if you think your life is danger), I don't think that the use of deadly force was necessary here. Even considering that Martin was attacking him before he pulled the trigger, he was the one who has the fault of allowing this to happen in the first place by following Martin, again against a directive from law enforcement to cease pursuit. He definitely should have been charged with something. Murder requires intent, and regardless of personal views there is no evidence that can prove in court that he went into this intending to murder someone. There is, however, plenty of evidence that he did kill someone, and that it was his actions that lead to this occurring. This is what manslaughter was made for, and would be perfectly appropriate. If you believe that Martin threw the first punch, then a not guilty verdict is plausible, or perhaps some lesser charge that addresses his actions that lead up to the conflict up to and including manslaughter. If Zimmerman did start the fight, manslaughter would be the minimum charge, at least in my opinion. I'm not a legal expert, but that's my appraisal of events.
As to who actually did strike first...I'm split. There's no evidence that proves one way or another, but there is some evidence that supports both sides. Martin may have done so because it seems he had the option to escape and chose not to. Perhaps he was under the belief he could not or should not, I don't know, but it is at least possible he could have thought he had a reason to confront Zimmerman and thus attack first. Zimmerman may have done so because he (however mistaken he was) felt that Martin was a danger in the community and wanted to apprehend him. Either way though it was just a pile of bad decisions that ended up in a death, and it was Zimmerman's fault. I don't think he committed murder (in the legal definition), but he should have faced some sort of punishment for what he did (killing a kid).
Overreached? You mean when they tried to claim child abuse? 
I think that was simultaneously the funniest and saddest thing I saw all trial.
