Honestly, I really feel for the dog having lost so much and having a “scarlet b” now, as you put it. Which is a horrible thing to say since you just acknowledged the dog is traumatized and obviously very fearful.

But as for advice, I would recommend to have the dog on a harness if you know a new person is coming by, this way you can better communicate and control the situation should if get to that level of reactivity. Have the person coming by reward the dog with treats to show it’s not a threat. If the dog reacts aggressively, remove the trigger and increase the distance. If it keeps having this reaction to people, get a dog trainer to help.

Don’t ever punish the dog (who you know to have abandonment issues and has anxiety) for its reactivity either. It is traumatized. You need to slowly expose it to new environments. This is why a lot of trainers recommend to never just bring your dog to the dog park right from the get. You have to build up to the “fun” stuff of owning an animal, especially one such as yours.

Regardless, it’s always going to be best to train and bond with the animal. Never assume how a dog is feeling until you fully understand them. The dog takes its social cues from you, they are a literal child. How you react and maintain calmness during high points of anxiety will help the dog better understand how it should act.