You are leaving out the Union/non Union dispute which was going on set. This young woman was non Union and the other set members were Union.
I predict that the investigation will reveal a Union member deliberately put real bullets into one or more guns in an effort to make the "scab" look incompetent.
I do not believe anyone could be so incompetent as to put real bullets into a gun that she knows will be fired at other people.
That didn't happen until they have astonishingly good evidence that it did.
Not outside the realm of the possible, but first we need to establish the responsibility of Baldwin, who was proximate. If the chain of custody was broken and another party snuck a live round it, how and why are pertinent but not exonerating of Baldwin..