My understanding is that the witness told the police that Martin was on top of Zimmerman, and that the physical evidence (grass stains on the back of Zimmerman's shirt, etc.) back that up. How they ended up that way, I don't know. The text of Florida's Stand Your Ground Act (codified as Chapter 776 of the Florida Statutes, the chapter's called Justifiable Use of Force,) says an individual who has a reasonable belief that his or her life is in imminent danger, or is in danger of 'great bodily harm', can use deadly force, no matter where they are, and they have no duty to see if they can escape the situation safely.
In a civil suit against this community, or if Zimmerman's prosecuted as a result of public pressure, the argument would be whether Zimmerman's actions somehow negate what otherwise appears to be a clear right to use deadly force under this 2005 Florida law.
That argument could arise in a number of ways, legally, at either the trial level (with a liberal judge) or appellate level. The issue of a community watch office having been told by the police not to follow a suspect, and whether that in any way could negate the apparent protections of the 2005 law, or any other alleged actions of Zimmerman, is likely an issue of first impression.
I have one question? Where did you get the weight of 190 pounds? I've seen him reported at 140 pounds everywhere. Is there a link to a football program, or any other source, that puts Martin at 190 pounds?