I say the chances the handloads will be THE deciding issue are close to zero.
Could well be. Consider this: The number of deaths in Explorer accidents involving Firestone tires (including those that were not necessarily tire related) was .0000027 per tire. Is that close enough to zero for you? But they recalled the tires, and Firestone ceased to exist as an independent entity.
Of course a person could show that police officers on the scene ask about what ammo was used and make that part of their report.
I'm surprised that you would wonder about that.
Practical Homicide Investigation: Tactics, Procedures, and Forensic Techniques by Vernon Geberth is considered to be the recognized protocol and is used by investigative divisions of major police departments throughout the world and the FBI.
Gathering ammunition evidence, documenting the type and manufacturer, and submitting it to the laboratory for analysis is one of the subjects covered. SOP. Same with any evidence on the deceased, from hair to wound information.
Now, my lay opinion, which is worth what you pay for it, is that if the evidence you are able to provide in your affirmative defense is overwhelmingly convincing (for example, unshakeable witness testimony supporting your story, a security camera, etc.), little will probably be made of the ammunition analysis and GSR evidence. It's when your evidence is less convincing that it would become a significant issue, as Fiddleback points out.
Remember, you are not contesting the fact that you killed the deceased. You are mounting an affirmative defense and providing evidence that the homicide was justified--that you were
required to use deadly force as an emergency measure to prevent imminent death or serious bodily harm to yourself or your family (castle doctrine situations differing, of course.)
and if they used handloads could report they received hostile attention from authorities.
How is that relevant to your being able to use GSR evidence in your defense? By the way, regardless of what is used, I might characterize being on trial for homicide as having "received hostile attention from authorities."