Michela, I think it goes without saying that you have a serious problem on your hand's now. I almost don't even know where to start. But, here goes.
The incident that happened with your friend when you were just talking is HIGHLY unacceptable. Incidents like these are why our breed has a bad name. Your dog is taking it upon himself to decide who to "protect" you from. Again, HIGHLY unacceptable. Diesel is so confused right now, he doesn't know what you want from him. That is why specialized training in personal protection is necassary with him if you are going to keep expecting to use him as protection. He needs to understand that you are the one who decides who he bites, not him. The problem is that you are trying to get him to act as a bodyguard but not taking the proper steps. I hope when he attempted to bite your friend you leash corrected his head off. He better have been in big trouble for that.
The next problem is your husband. Earlier when I suggested you stage a scenario with an experienced helper with Diesel what did you say? You were worried it would damage your dog's relationship with your trainer. Well, what do you think happened when your husband came in like a fool waving his arms "to see what he would do"? Now, your dog thinks everyone is an enemy, not just strangers but loved ones like your husband. It goes without saying that this is going to take some serious work to undo what has been done. I can't believe your husband enticed your dog take a bite out of him....
You need to decide right now what you are going to do with Diesel. Is he going to be a personal protection, or is he going to be your pet. Stop riding the fence and make a decision. This is completely unfair to that poor animal. If you can't trust any trainers in Italy to teach your dog to do protection than you need to stop confusing him and make him a pet. Maybe you should invest your time into a concealed weapons class and get yourself a gun. That way Diesel can enjoy being a dog and not have to worry about protecting you from your husband. Good luck