I agree that one doesn't need Zapruder's camera to make the case against Oswald.
But Zapruder's camera was silent and it did not have great resolution. If Zapruder's camera had sound, better resolution and a steady tripod, the zfilm might have perfect recall. In any event, you are relying on witnesses to support an early missed shot. The zfilm does not show that without an interpretation based on preferred witness evidence.
The Z-film alone gives us clues as to when the first shot was fired without using any witnesses. We see JBC turn to look over his right shoulder at Z164. Here's where a CORROBORARED witness can come into play. He said he turned in response to hearing the first shot which he recognized as the sound of a high powered rifle. The film and JBC's statement jibe. Without JBC's statement it would be hard to determine that his turn to look over his right shoulder was a reaction to the first shot. The fact the film and his statement jibe strengthens the probative value of both. In addition we have Rosemary Willis running alongside the limo. She begins to slowdown during the Z160s and when she comes to a stop, she turns and looks back to the TSBD. She later said she did that upon hearing a gunshot. She didn't testify before the WC but was called by the HSCA. None of this is proof positive as to when the first shot was fired but taken as a whole, it is probative of a shot prior to Z164. If we add to that the blurring and frame jump in Z155, it is a strong indication of a shot around Z147-148.
If we don't read the reactions of JBC and Willis as evidence of a gunshot, there is nothing in the Z-film that would tell us when Oswald's missed shot was fired. That is why the WC never determined which shot missed. They allowed that it could be the first, second, or third shot.