seems to me that the AD is in a really bad spot, but i'm not a lawyer, so...
He's already plead guilty to a lesser charge:
"In exchange for a suspended sentence, the film’s first assistant director, Dave Halls, who handed Mr. Baldwin the gun, agreed to a plea deal, admitting there was sufficient evidence to convict him of negligent use of a deadly weapon." (NYT, not CNN).
The other two did not plead and here we are. I think the DA has political ambitions and wanted to see her name in the paper on this. I doubt the judge finds probable cause for Baldwin, possible criminal negligence for the armorer, but not involuntary manslaughter. It's a flimsy case resting on "industry standards or practices" and not legal requirements. Industry standards are great in civil cases, but are almost useless in criminal cases. The DA was probably frustrated because the 15 month investigation did not reveal who the real criminal is: whomever brought the live ammo to the set.