Herein is your problem. You put all your faith in fallible witnesses and reject what the Z-film tells you unmistakably. The Z-film is more reliable than 20 witnesses because it has perfect recall.
We don't have to look to other events to find more than 20 witnesses getting something wrong. More than 20 witnesses said all the shots came from the GK. More than 20 witnesses said all the shots came from the direction of the TSBD. They can't both be right. So there's your example of more than 20 witnesses getting a pertinent fact wrong.
First of all, determining direction of a sound is not a simple observable fact, especially in Dealey Plaza. Second, the apparent direction depended on the location of the listener. Third, many of the witnesses said they couldn't tell but still offered a a guess. Finally, this is not an example where there were zero who observed it correctly.