Oh, I'm really, really sorry you're going through this mess. But I have to say that at 9 years old and with some health issues, if you choose not to keep her, please do bring her home, treat her good for a few days, then take her to your own vet to have her euthanized.
You only want to place your pit bull with a reputable rescue (otherwise, lord only knows what could happen to her) and a reputable one will not adopt out a dog that has a bite history. Even if it does not seem to have been a major incident, it's a huge liability that a good rescue knows better than to take on.
You also do not want to put your poor old dog into a shelter, where she's very unlikely to be adopted out--and if she is, again, you have know way of knowing what kind of home she'll end up in and how the kennel stress will affect her.
And lastly, you really don't want to let the poor old girl get put to sleep iin a shelter, alone and confused and abandoned. Please, take her home and have it done at your own vet. It may be painful for you to have to deal with it, but it'll be much less painful for your dog. Really, it'd be selfish to not do this just to save yourself the heartache.
If she's never shown aggression to humans before, you can (and probably should) try to keep her yourself, but if your husband refuses (which is really not fair to the dog after all this time, if she has been as good as you describe her to be), I'd strongly urge you to have her put to sleep yourself.