Unless he meant to shoot her, it was an accident. I don't see this as a criminal matter. What does society gain by anyone going to jail for this? It is much more a civil matter in my mind.
Did they ever actually determine how the live round ended up in the gun?
I disagree. There are processes and rules that were ignored and it ultimately cost a young women her life. Baldwin was the person running the operation and was responsible for the complacency in regards to safety. Early in the reporting, it was said that the armorer was not allowed on the set and placed the guns outside on a table where the AD picked them up and took them inside.
When you have a leadership team that hires an inexperienced armorer and then interferes with her ability to perform her job, while taking a lax approach to safety, it sounds like criminal negligence. The armorer is part of the problem too if she allowed this to happen without raising the red flag on the whole set.