I am so sorry to hear about what happened to Lucy. My name is Rebecca; I am a veterinarian and will do my best to help. It appears that Dr. Scott is not online right now, can I help you?
Do you happen to know where the fracture was, which vertebra was broken? Knowing that will help to know if this is permanent. If it was a tail vertebra bone, she may get better; if it was a vertebra bone in her lower spine, she may not.
Expressing the bladder is not difficult, but is easier to show you than to describe. You need to feel for the bladder inside her lower abdomen, it would feel like a smooth round object anywhere from the size of a golf ball to a lemon or even an orange. Then you grasp in your hand, with your hand around her lower abdomen, and gently push until the urine stops flowing. I would do this at least twice daily. Some people do it with two hands, on on each side of her lower abdomen.
She may also need gentle enemas. If she can't empty her bladder on her own she may not have control of her colon, either. Do not use enemas meant for people, or children or infants that you can buy over the counter. They contain phosphates which are dangerous for cats.
The problem with nerve damage is that the nerves heal slowly. You may have to care for her for weeks, even months, before you see any nerve function return if it does.
If you know where the break in her tail or back is located, that will help somewhat to predict whether she will get this function back.
Let me know what else I can answer or help with, or if you prefer to wait for Dr. Scott. If this was helpful, kindly consider rating my answer (always appreciated!); you can still get back to me with questions and updates after rating. Rebecca