No, it was not. The fact that he didn't double-check is, especially since he was also the producer. I'd also heard, and this might be totally unsubstantiated rumor, that in the interest of "authenticity", AB wanted "real" guns that fired something as close to real bullets as possible. If that's true, that would be on him as a producer.
If the prosecutors had that evidence or testimony, they wouldn't have hidden the live ammo from the defense. I imagine they hid it because they were afraid they'd track the origin of the bullets to someone other than AB.
I disagree. *He* ultimately was responsible for hiring the licensed armorer he "thought" was safe. IIRC, it was only her second time on a movie set and while yes, you need to actually work to gain experience, he could've hired someone with more experience or hired someone to double-check her work.
He's not an ammo expert. If she had the proper licensing, he might have a civil responsibility but not necessarily criminal one. And I do believe he reached a settlement with her husband.