I don't need Helen Markham. Oswald is a cop-killer; no need to rely on Markham at all.
No need to rely on the only person who witnessed the crime?
Shells were found by Barbara Davis and Virginia Davis and turned over to law enforcement. The chain of custody for these two shells is perfectly intact. These shells are evidence.
Prove those shells were found at the crime scene. Or even the same ones found by the Davises.
They were linked, through ballistics, to the revolver taken from Oswald to the exclusion of any other weapon in the world.
Prove that revolver was taken from Oswald.
Whether you like it or not, these shells would be used to convict Oswald.
That's easy for you to say.
Unless, of course, you FINALLY have something to support your presumption that the shells aren't authentic. Do you?
You can't demonstrate their provenance. That's enough.