Re: the partially open slide ...
There were gunshot residue patterns and cuts on Trayvon’s hands, which seems to indicate a struggle for the gun. If Trayvon’s hand interfered with the slide mechanism, that could explain why another round was not chambered.
Can you source there were cuts on TM hands?
If Trayvon had succeeded in getting control of the gun and had shot Zimmerman, nobody outside the immediate area would have heard about the story. What are the chances that Trayvon would have expressed sorrow to Zimmerman's parents for their son's death? He might have tried to claim self-defense but it would have been difficult since it was Zimmerman's gun and Zimmerman had called the police dispatcher.