Structure safety is locked green. Typically if you’re the pilot and someone shoots you, you can shoot back. I’m not sure how this works with structures, either, but there’s no harm in trying to fire on the aggressors. At worst you won’t be able to fire.
The answer is no. You must be in that corp and participating in the war to be able to shoot the guns at war targets. A high-sec structure is also worthless against suspects. Only useful against war targets.
Meaning you can’t shoot suspects in a structure.