If antibiotics are no longer efective, surgery may be the only alternative. Surgically removing the infected tissue may cure the snake, but this may not even be an option here, and only your vet would know if surgery might help. I suppose it's possible that the snake's immune system could eventually clear the infection, but the infection is still present after 6 months despite the care given by you and the vet. I wish you good luck. Please let us know how this ends, good or bad.