If Zimmerman tells his story and it matches what evidence can be verified - for example, where the eyewitness saw him getting pummelled - and can convincingly show he actually was on the way back to his car, having stopped following Martin, he ought to be acquitted. (I sort of picture an overhead map view with a timeline, with the locations of the path he drove his car along, where he walked after getting out, where he parked his car, where the eyewitness saw him, etc. all marked, referencing the investigator's notes, with a narrative to match his statement from the night of the incident.)
On the other hand, if the verifiable evidence doesn't match his story, he has a big problem.