" Only (a) what actually happened and (b) what Zimmerman reasonably believed are in point (to an issue of self-defence). The contents of a phone about which Zimmerman knew nothing illuminate neither."
I think you are wrong. The contents of text menages might well provide supporting evidence for what happened. If they stated "If anyone follows me, I am going to run like hell!" the defense case is weakened- if they stated "If anyone tries to stop me I'l beat the shit out of them." the defense case is strengthened.
When the defense claims that MARTIN attacked Zimmerman ( leading to the shooting), evidence that MARTIN had intentions of violence would certainly be significant evidence, and bolster the case of the Defense. I do not know if that is the case in these texts, but it could be, and thus the need for the JUDGE to rule on admissibility as evidence, NOT the prosecution.
IF Martin attacked Zimmerman, it would reinforce a claim that Zimmerman was in fear of his life, and support the rest of Zimmerman's testimony.