Motive is the big issue. What would somebody hope to achieve by either a) loading a live round into the weapon or b) putting a live round into the box of dummy rounds? Any number of people had means and opportunity.
As far as HGR goes, the physical evidence is now hopelessly tainted (thanks to DH's reported order to unload the gun, and HGR complying) so a prosecution that survives the reasonable doubt test is hard to visualize. Civil liability is something else.
If only HGR's DNA is found on the cartridge (per your account of DH telling her to unload it), that should make the sabotage defense impossible or unlikely. If there's stranger DNA on the cartridge, it should match someone who had been on set (I believe most ammo is machine packed and not manually put into cartons - but the FBI will know the brand and the process).
I just hope there's DNA on the spent bullet casing, that's the one that needs close examination. Unfortunately, it will certainly have the director's DNA and HH's DNA on it as well. So it should be just those three people.
"He’s an actor, not a trained safety specialist. As far as I know, he’s not an avid gun handler in real life." AB does, however, handle guns as a highly paid professional actor.
If Hanna checked the rounds in the box and the gun, What, exactly did she check for?
There were three (I think) guns on the cart. Is it possible that only one was checked and that one was mistakenly not chosen for the scene?
Any un-used ammo (blanks, squibs, dummies) left after production would have been taken by the Armorer and sold to the next production that needed supplies. Additional revenue stream. Firearms were no doubt rented from prop company.
That's a really good question. One was a plastic gun, one was an operational Colt 45 replica, and one was an inoperational Cold 45 replica. For the purposes of the "technical rehearsal" the inoperational Colt 45 should have been used. Indeed, depending on what was going on, that should have been the only gun on the cart, as surely they weren't going to be firing blanks at such close range later in the day. OTOH, if this production was so on-the-cheap that it could afford any digital after-effects, then maybe they were going to try and get the smoke effect that a blank gives in that afternoon's shooting. I suppose they would have worked out where everyone would stand and perhaps operate the camera remotely. Alec should have been given a specific target to aim at, and no one should have been in the line of fire, period, full stop.
I think it was approaching chaos on this set. Alec says the crew was basically whining about travel time and that no one was working too many hours. Crew members saw it differently. Alec says that the Bonanza Ranch is "halfway" to Albuquerque, so it's not a big deal - but it's 51 miles from the ranch to downtown Albuquerque, and 45 miles to North Albuquerque (which is hard to book in October, because the Balloon Fiesta participants come in early and often stay an extra week). It's 24 miles from the ranch to the ritzy part of Santa Fe, but there are hotels much closer to the set...
At any rate, several sources say that crew members were sleeping in cars on set...3 incidents involving blanks being fired when it was not intended...live rounds on set...DH with a past history of safety problems. Big Yikes.