JKR we have been around this mulberry bush countless times. Neither expert said that casing had any effect on the firing of the gun. And no, they don't last much longer.
The fact that you are convinced it wasn't an accident doesn't make it fact. Neither expert ruled out the possibility of a hangfire. Neither expert could explain the bulge in the bullet. That can happen as a result of hangfire. Hangfires are more common with old ammunition. The ammunition was from 1953.
Nothing was proven either way. Benefit of the doubt goes to the accused.