The rabbits were finally taken to the veterinarian yesterday. As we thought, the affected ones all had mites. They were treated and our Silver Fox buck did not need to be euthanized. That alone is a blessing as my associate is particularly attached to that rabbit. She actually cried when we took him to the vet, because we believed he wouldn't be coming back.
He still has some battles left to conquer. His softened toenails were caused by the mites entering the nailbeds, he has (slightly) maloccluded tooth and a slight fracture in his back. All in all, however, I believe he will recover fully and, once again, be himself. Perhaps next time my associate will listen when I insist a rabbit needs to visit the veterinarian. :)