A muzzle can make an aggressive dog worse. I use it when I take ChiChi out of the house for her sake as well as the sake of everyone she comes into contact with. I have no choice. If you can find a trainer or behaviourist to work with, that will be your best bet. This is not his "normal" behaviour, so it may very well be something you can fix. It sounds to me like he is missing the ones he has lost from his life and is "acting out" because of it. He doesn't understand what happened or why they "left" him. If you are totally confident that you can control him, are there people you can have come over to "condition" him to? Is he this way with everyone, or is it only certain people? You need to put him on a very short leash and make sure you can hold him, no matter what. Have them come in and as soon as he does anything that is not acceptable to you, give him a firm correction and tell him NO, or whatever you choose to use. Have him sit facing you and make him maintain eye contact. Have them move around, come closer to you, walk by him, etc. As long as he is focused on you, reward him, treat, petting, whatever. Do this often. Take him for walks, where he will see people, but not interact with them, maybe by a fenced park, or somewhere that you can control his exposure to people. If you need to, warn people away from getting too close. Walk towards people and watch him. As soon as he starts to act in any way aggressive, correct him. You need to teach him that any behaviour that you do not approve of will not be tolerated. It may take months to get through this with him, but if you work on it diligently, it should help. Watch him closely with your JRT and if he begins to act aggressively, ears laid back, stance changes, etc. correct him. You may need to keep a "handle" or some type of short leash on his collar, to give you leverage in controlling him or pulling him up sharply, if he goes to attack. I did this with Max, when he was a puppy, to stop him from chasing my cats. He wore a 6 inch leash on his collar for months. I understand your feelings, as far as working with him and not putting him down. ChiChi will have to be put in our bathroom, with the door locked, most likely, her entire life, whenever anyone comes over, but that is the price I have to pay for having her in my life. She is my cuddly, love bug, but does not want anyone other than my hubby or myself in our homeor even in her presence. We have accepted that and deal with it. I don't think your situation will be that bad, if you start working on it now. Good luck.
Kathy