the armorer on set was the daughter of a long time industry professional (nepotism?)
Her dad is long retired and she did not get the job through him, so no, not nepotism. This was a matter of the producers being greedy, cheap, and stubborn; multiple experienced armorers that they tried to hire turned them down because they wanted too much work for too small a staff. Instead, they hired someone who wasn't so experienced and agreed to try doing it their way.
and she was reportedly cussed out by Nick Cage on the set of another movie a few months prior to the incident for her incredibly unsafe practices which caused him to walk off set.
It wasn't super unsafe. She was test firing "too close to the set" for Mr. Cage's taste and he claimed it was hurting his hearing. He stormed off and complained about it, but note that she *wasn't* fired.
Now the woman claims she "has no idea how live rounds appeared in their ammo supply" and proceeds to blame the producer.
The assistant director involved in the incident is the one who actually handled the firearm and falsely declared it "cold", *plus* he's the one who has an actual history of unsafe gun handling practices up to and including being fired on the spot for an incident in 2019 that in fact sounds very similar to this one -- he declared a prop firearm "cold" when it was in fact hot, leading to a crew member being shot and injured.
Isn't the armorer supposed to be the only one with access to the armory? Aren't they supposed to keep track of the ammo supply?
That's industry best practice, but at the end of the day, the armorer is an employee and the producers are the ones who call the shots, especially with someone who *isn't* enough of an established name to make a walkoff stick.