Absent a new mystery eye-witness who is absolutely reliable we will never know the facts.
I am not willing to accept Zimmerman's version as anything but CYA. I would feel the same if the roles were reversed.
The emergency medical people and the police who saw him minutes after the shooting should be able to testify to what they saw, I imagine. There were two or more witnesses who heard and perhaps saw the fight--maybe not all of it but at least some of it. The police should know who they are and Zimmerman can compel them to testify (at least the sixth amendment says he can--but maybe that doesn't apply since it is the state of Florida bringing the charges rather than the federal government).
Fortunately, I've never been a plaintiff or defendant in any criminal or civil trial, and I've never gone to law school, but I think the defense is supposed to have access to any exculpatory evidence. Of course if the prosecution hides it and they don't know about it, tough luck for Zimmerman.