Only two people were at the fight when it started, and one of them is dead. The story given by the other lines up with the physical evidence, and as a result, is believed by sane people.
Bottom line however, is that it doesn't make a difference. It's almost like you are arguing whether Treyvon was wearing his Sponge Bob or Power Ranger boxers. Irrelevent to the matter at hand. Even if Zimmerman threw the first punch, it doen't not give Treyvon the right to kill him. Once Zimmerman was on his back screaming for help while Treyvon was bashing his head against the concrete, Martin become the attacker and Zimmerman, fearful for his life, has a right to defend himself.
You are correct in that there is no evidence as to who initiated physical contact. It doesn't matter. You're entire argument is based on this one tidbit but your anal-retentavity does not permit you to understand that it's irrelevant.
If someone pushes me I can't beat them to death with a crowbar. If they punch me, I can't shoot them. If they knock me to the ground and keep coming at me the situation changes. When they are on top of me bashing my head against the concrete, or standing over me kicking my head, they have escalated beyond reasonable force and have to bear the consequences, like being shot dead.
There is no eyewitness other than Zimmerman, but there is evidence on the point. One or the other in fact threw the first punch, or made the first move that is credibly taken by a reasonable person as an attempt to make unwelcome contact.
I agree with your legal analysis, but the case also involves Zimmerman's credibility, because nobody saw the details in the 5 seconds leading up to and including the gunshot. So, we are stuck probing Zimmerman's credibility in light of his account of events. Zimmerman says (according to his brother and father) that he was coldcocked without provocation*, and at physical disadvantage from that point until more than a minute later when he discharged his gun.
If you find Zimmerman to be a liar about being coldcocked without provocation, it'll diminish your willingness to believe him when he claims he had no choice but to shoot, for whatever specific reason (choked out, Martin saw the gun and moved to take it, etc.), a minute after being put at physical disadvantage and seconds after having the presence of mind to scoot his body so his head wouldn't hit pavement again.
The only person who will tell the details of the end of the fight is Zimmerman, and you'll either believe him, or not. The forensic evidence does not contradict his account, it supports it. But the forensic evidence does not foreclose all possible scenarios.
* Provocation as a legal term means at least an assault (talking and/or moving in such a way that a reasonable person would think the use of physical force was imminent), or a battery. Following, trash talking, asking questions, begging for money, and otherwise being a nuisance are not "provocation" in the legal sense.