You are right, and I agree with what you say. However there is one mitigating circumstance, and this falls back onto Baldwin in his Producer role, she was doing double duty on the set as both Head Armorer and as Property Master, and that is why she wasn't personally on the set that day with the firearms. She was off on another set dressing it with props.
I did say Alec was liable as producer in the company who hired her.
I dont think thats a mitigating circumstance. The armorer also has the job/responsibility of physically securing the weapons. If she isnt there, no one should be able to get to the weapons, because then, the armorer cannot declare them to be safe/clear, or ready for a scene. Something in the chain of responsibility that starts with her, was off.