It has occurred to me that you really don't understand what the prosecution is trying to prove. The syg has nothing to do really with either of them. The key to the prosecutions case is that Zimmerman acted with depraved heart. That is the only way they could win. In order for them to do that, they have to disprove that Zimmerman was defending himself. If Zimmerman believes his life is in jeopardy, then he may act with deadly force.
I think most reasonable people would say that a person who is on the ground, getting hit from above and there is a gun brought into the scenario deadly force is justified.
I think you have been distracted by their tactic, just like the defense. I think the prosecutions end goal all along has been a manslaughter conviction. While this may have been murder 2, I doubt the prosecution really thought they could prove that in court. The murder 2 charge will hinge on the jury's interpretation of zimmermans calls to police. If they feel he really did have ill intent based on the words he used and the actions he took, that's the case.
For manslaughter all the prosecution has to do is prove this wasn't self defense.
A reasonable person would feel threatened by someone following them. This would be heightened in an area that was "plagued with crime" as Zimmerman described. And they would be further escalated when the person who was following you in a car came towards you on foot after you had run from him. And finally, if that person who was following you reached to within inches of his firearm instead of identifying himself when asked why he was following you, a reasonable person would feel threatened. So yeah, SYG does apply for Martin it it applies to zimmerman. The jury will have to decide if Zimmerman can claim self defense after he followed Martin. That is unless the judge tells the jury that Zimmerman can or cannot during jury instruction.