I just don't have the patience for this. But what the hell, I'll give it a go.
Cranes said he'd give an opinion about whether the shooting was in self-defense. When is a shooting in self-defense? When the shooter reasonably fears death or great bodily injury and reasonably thinks that lethal force is necessary to protect himself from it.
Hey, that sounds pretty technical. Cranes probably even teaches what "reasonable fear" means, what great bodily injury is, when deadly force is necessary. Thing is, though, the judge is the one who will explain that to the jury, not some blowhard CCW instructor. And once the jury knows it, they know everything Cranes does. He can't testify about whether it was a good shooting because he adds nothing.
"Seems to me that if the prosecution wanted to make the point that Zimmerman was trained not to pursue or follow a suspect while carrying, the best person to make that point would be somebody who provides that training" isn't what Cranes said he'd testify about. It does have a little to do with self-defense, at least in this case; the prosecution will argue it's relevant to Zimmerman's motives and subjective intent, probably get it in there and hope the jurors are confused about why it matters. And some of them probably will be. But, again, it isn't what Cranes said he'd say.