Thank you for that information. It sounds like you have a fear aggressive dog and these type of issues in a dog can only be taken so far and here's why. A fear aggressive dog will learn to trust those that are consistent in their lives, however meeting someone new there will still be caution in the back of their mind and if that person is calm, making no fast moves toward the dog , they may do just fine. However, if that same person makes a sudden move that the dog is not expecting then that person becomes inconsistent to the dog which triggers the fear. Usually dogs that feel they have no way to escape will bite. They do sometimes give a warning growl or bark, but if the human pays no attention to it, then they will lash out with a bite.
My suggestion to you is to always tell a person to not approach your dog at all. You have had two incidents of a bite, that will only go up with a fearful dog, because humans are unpredictable and do not head the dog's warning.
At this point it is up to you to protect your dog from humans.
There are three things that humans do that set off challenges to dogs. Looking at, talking to, reaching out to pet, are all challenges to a dog. A perfectly well balanced dog will not mind these challenges, but a fearful dog is going to magnify them 100 times and that is why they act out.
Is this understandable to you?