From what I've read online, if an actor cleared a gun handed to him by the prop person/armorer, it would be considered a violation of union rules and could result in a halt to production.

Of course the unions had already walked off the set and been replaced by non-union workers on this movie. The walk-off was for repeated safety violations, including guns that had accidently discharged at least twice. Go figure. But as executive producer, Baldwin is ultimately responsible for that.

There is also a lot of discussion online as to whether guns are ever pointed directly at another actor. Some claim it's all editing tricks and camera angles and the close ups with guns held to head etc. are done with rubber or plastic fakes. Not sure if I believe that's always so. The closeups maybe but not the rest.

Legally, as you said, Baldwin is guilty only of involuntary manslaughter. But I imagine the civil suits will sprout like weeds.