I do not think the rabbit needs to be euthanized unless at some point you find she is painful or uncomfortable while performing her necessary range of movement to sustain life. It's all about quality of life. I would keep a close eye on her and ensure she is eating, drinking, urinating, and defecating properly and regularly. There is always a concern with spinal problems that urinatiion and defecation are affected, so you will need to watch for this. As far as special needs, I'd just ensure that she is pain-free and can easily get to her food and water and gets out of her cage for some exercise. For nutrition, I'd suggest sticking with a good pellet diet. You are very right in your thinking that a docile animal could be illiciting signs of illness. Keep this in the back of your mind and watch for changes in eating and drinking habits, behavioral or activity changes, and changes in frequency or appearance of defecation and/or urination. Remember if at any point you are concerned she is in pain, check into pricing out a physical exam by a vet near you.