What degree of certitude is needed to bring SYG into the picture? I’d say more than that which is had by having a story that varies like this. We can’t tell if this is a lying vengeful thing or crazy or what, so hence a trial where the prosecutor gets to try to prove it was a crime.
It's "self defense immunity", not SYG. She would need to show that it was self defense by a preponderance of the evidence. That's a higher standard for the defense. At a trial she would only need a reasonable doubt. But if she won the ruling at the hearing then she wouldn't have to stand trial.
All SYG means is that in a self-defense case you have no duty to retreat before using force. In some jurisdictions you do have such a duty. All other elements of self defense are still required. SYG is not a different kind of self defense claim, and there is no such thing as a SYG hearing.