A prosecutor [because that's what Ball was in this case] calling something a "conclusive fact" doesn't make it so. But there is no surprise that Ball would say that. He wrote the chapter on Oswald's guilt, so what else was he going to say? He also said that Oswald was guilty beyond a reasonable doubt, yet he failed to convince the majority of the American people. So, Ball's opinion is just that; an opinion.
This is the same man who called Helen Markham an "utter screwball" yet at the same time considered her testimony to be reliable. That alone tells you all you need to know about Joseph Ball!
Btw LNs whining about a so-called "impossible standard of proof being applied to evidence of Oswald's guilt" is hilarious and sad at the same time. It's like a prosecutor complaining to the judge about the jury because his arguments (which he himself finds amazingly powerful) fail to convince the jurors. It is in fact an implicit recognition of the weakness of the prosecution's case.
The fact that those highly skilled lawyers of the WC were willing to blindly accept, without any kind of authentication, a couple of photocopies (which even Lyndal Shaneyfelt admitted, during the mock trial, can easilybe tampered with) taken from a microfilm that has since gone missing, as a so-called "conclusive fact" only exposes the desperation they had to wrap the case around Oswald as tight as they could. No competent prosecutor would have dared to present something as pathetic and weak as this in court.