Very sorry for your loss. I can only imagine how heartbroke you are.
Is there a law that prevents the ambulance company to wait till the scene is clear before they respond?
I'm not sure if it's a "law", but at the very least I would speculate that there may be an agency protocol that the emergency personnel are not to go on scene until it is clear, that being for safety of the personnel. EMT's are not trained in law enforcement, and had a gunman still been present, an EMT being shot isn't going to be able to treat a patient by also becoming one.
He was shot in the chest and possibly could have been saved.
Unfortunately medicine is not exact science, and with something as serious as being shot in the chest, it's just as possible that even if the scene had been clear when personnel arrived, no guarantees.
When they took my 16 yr old son to attend to him, the EMT was harrassing him, verbally assulting him and twisting the arm he was shot in.
I'm NOT trying to sound like a jerk, but was this "witnessed" by yourself? Otherwise, I'd speculate this would be a his word vs. their word. IF it was documented by the doctors that the arm being twisted caused permanent damage AND not just being shot alone, there might some remote unlikely possibility of a case. Not very professional if that is what happened, however, what you could do is contact the ambulance agency and file a complaint with the supervisor. If that particular EMT has a history of being abusive or complaints of not being professional towards patients, then at the very least, that could be addressed at the ambulance agency level. For all you know, the guy does have complaints of being rough, etc.
Also... one last thing... I suggest REMOVING your email address from your original post. Spammers love harvesting email addresses, and any contact here will be through these forums.