Excellent post, I wish I wrote as well as you. My objection here is to those who claim that self defense is not affirmative, and must simply be assumed once one mutters these magical words.
The defense has presented their claim and some evidence to support it. The prosecution has poked some holes in their story. The kid is dead, and it will be up to the jury to decide if they believe Zimmerman's version of events. If they don't, if they do not believe that Zimmerman is telling them the truth or that his claim does not match the evidence, they should find him guilty.