buy a muzzle, use when anyone is visiting, or when you are out and about use it,
You can with very carful training change the reaction of the dog,
but as a responsible owner you must never fully trust the dog again.
I have had in the past had "dangerous dogs" because I acknowledge their limitations, I was able to provide a safe home, and re train their responses, it does take a lot of work, and understanding of k9 responses,
I train dogs for purpose , the dog you have has not had the correct training at the right development stages, you need to adjust to this, you have done exactly the right thing by acknowledging this, most owners pretend it hasn't. (you came close with the flapping trousers), really anything can be a trigger,
if you get the cage type muzzle , they can still sniff, drink, bark, but can't bite, which will help you all relax a bit.
This will give you the time to rebuild the dog's confidence and behaviour.
You need a really good trainer, who will help you work through a program,
The first owners of this dog, had no concept of dog training, they allowed the dog to mouth them, this is the very first rule we teach puppies, you get shunned if your mouth touches a human, this dog never got this message.
so you need to find someone who really understands K9 development, you have to go back to beginning,
start with basics, the dog only get attention, when it's bottom is on the ground, full obedience training,
I also suggest you make sure that you walk this dog a lot more, never the same walk twice in a three month period, walk every inch of the surrounding area, all the nooks and crannies,dogs want links in their head of whole area, the same walks are like reading the same issue of a magazine over and over,
dogs build a smell mind map, their confidence is built on this, cover both sides of all routes,
Don't think of muzzle as negative, a lot of vets and dog behavioust would like to ban leads and make muzzles compolsery People unintentionally send negative message down leads.
Don't worry you can sort this out, and save this dog, find someone in your area who will help you build a program for the dog.