Ah, I see. The problem is that it need not be proven that he was attacked unprovoked by Martin, because he does not need to be exonerated. Rather, the opposite is true - there must be probable cause that that Zimmerman attacked first to establish this.
The mere fact that it is unknown who attacked first does not establish probable cause for an arrest of Zimmerman.
If it were the case that Zimmerman was the initiator and aggressor prior to the 17 year old allegedly hitting Zimmerman, and Zimmerman shooting the 17 year old, does that change the issue of probable cause to effect an arrest of Zimmerman?