The burden of proof is on the state to prove beyond a reasonable doubt that Z killed Martin. At this point, there's no doubt he did. Z has to prove that his actions were justified or excused in some way, the self-defense theory. HE has to prove it was self-defense. The witnesses saying that Martin was on top of Z and beating Z helped him to that end. Them changing their stories doesn't.
Their credibility is essential if they were going to testify to facts that bolstered his self-defense theory. They weren't initially going to help the state's case anyway.
If jurors start basing their verdict on what they heard in the media, it will be a monumental failure of the attorneys and the judge. I know it's hard for you to believe that jurors take their jobs seriously, but most of them do once their empaneled.
The important part of the self-defense theory isn't that his life was threatened... both of their lives were threatened. But if there are witnesses out there who say that Z was the instigator and had the upper hand in the skirmish, then it's going to be harder to prove self-defense. It's not unusual for a killer to have injuries, and it's not dispositive of a murder charge.
Well, to be fair, there are different levels of burden of proof. IIRC the self-defense burden of proof is preponderance of the evidence, not beyond a reasonable doubt. It might be clear & convincing though. I don't know off the top of my head... would have to research it.
