As I say, I am so sorry you're going through this, it does rescue, dogs, the whole fucking shebang no good whatsoever and these 'rescues' carry on because frankly, its a good earner for someone, somewhere!
The 'attack from behind' MO. is a classic fear behaviour, someone facing the dog poses a much bigger threat. They turn away or aren't looking, the dog still feels threatened but now has a smidgen of confidence, just enough to have a pop.. and some will moderate themselves and it'll be a muzzle punch to the back of the leg (closed mouth, exactly as it sounds) and then some are all the way up the other end of the scale, jumping up to bite fully.
Very typical of dogs who have experienced aggression from humans or other dogs, they've learned when others are at a disadvantage (back turned) but are so scared that no longer feel any relief from someone turning their back (normal dog would find this a de-escalation of threat), they just see the opportunity to drive the 'threat' further away.
It typically does get worse before it gets better as the dog unpacks things that have been suppressed, grows in confidence in some respects but not others, starts to communicate more so behaviour gets 'louder' as it were... and how long that 'getting worse' period will be... no one can say!
And this is why rescue dogs should be in a well equipped foster or rescue centre to be fully assessed before they go out to homes.
The advice for your husband to give high value treats is also massively over simplified and done wrong, increases danger. Him having the high value food (assuming the dog is capable of seeing those as rewards) could increase arousal and stress.
If he is offering/luring the dog closer, which is most peoples natural instinct to do, then the dog is pressured (and any dog who has gone without food will feel that pressure enormously) to approach - food eaten = no distraction, too close to threat = bite!
At the point where I'd have someone else giving food (and that would not be now!)... I'd be asking them to throw treats past the dog, so that the dog is encouraged to move away, repeatedly reminded that moving away is good and is safe. It would produce feelings of relief (increased distance from threat) as well as positive reinforcement classically conditioning the idea 'this person is good news'. Scared dogs, even ones who bluff confidence, forget they can move away!
Im not telling you all this because I think you can or should attempt to fix this dog, just to help you understand a bit. Making tough decisions is always easier with a bit more information I think!