Shadow of a doubt is the wrong standard.
It doesn’t really matter who was yelling for help. If GZ feared great bodily harm or for his life he was justified in shooting. There is no requirement of calling for help before exercising his right to self defense.
According to Jeantel TM had made it to his apartment and then went back to confront GZ.
I agree it was unfortunate for both of them. Obviously TM because he’s dead and GZ because his life is utterly ruined no matter the verdict.
If the jury had received information about the full self-defense statute, the yelling would be significant, in that it would imply that even if George had (hypothetically) started the altercation, he was doing everything possible to retreat from it, thus any question of who started the altercation would be moot. Had GZ not been yelling, he might (very vaguely) conceivably have been a voluntary MMA-style combat participant. The yelling means that he clearly was not, and TM could not have plausibly believed that he was.