The armorer job is to make sure all weapons are clear and safe to use which she didnt...thats why she is there. Alec Baldwin is not a weapons expert...he's an actor. He's not going to know what to do which is why he hired an armorer.
He would not have been convicted anyway, this was done as a political stunt
To conclude
I know exactly what the Armorer's job is, what you're not understanding is that whether you're on a set, firing range or even dry firing in your home, the last person to handle the weapon is the last one to make sure it's clear....this is basic. If i'm dry firing in my home and I pass the gun off to my wife for her to practice, I EXPECT her to clear that gun again. If I go into a gun store and I want to buy a particular gun and the guy hands me the gun to look at, hold, etc, the FIRST thing done is open the action into slide lock and make sure the chamber is clear both by physically looking into it and sticking your finger in it. If you handle guns, this is automaic.
If the armorer knew there were live rounds on the set they should have been seperated and placed OFF SET seperate from the dummy rounds in a safe or at least locked away. Yes, it's the armorer's job to make sure the guns are functioning and right rounds are used, but again, back to the safety rules for everyone ESPECIALLY last person handlng the firearm.