Haven, I am not sure that your statement about just treating for EC is true. I doubt the bunny has EC because of the sudden onset. In addition, the treatments for EC will not work within a day and can be quite expensive. If you want to properly treat EC, you need to do a blood test to determine the level of EC parasite in the animal, and then treat with either drug for several weeks, and do another blood test to see if there is improvement.
It sounds much more Haven had nerve damage to me. That can result from a fall or other traumatic injury and may not show up on x-ray. Sometimes anti-inflammatories help; sometimes they do not. It is likely that the Metacam Haven was given was what helped him heal and regain full leg function; not the Panacur.
alien, have you looked at our lists of rabbit-savvy vets? Depending on your location, we probably have a member-approved vet in your area. If a bunny suddenly presents with hind leg paralysis or reduced function, the most likely cause is nerve damage or spinal fracture, not EC. EC is a long-term, progressive disease. I would have the bunny x-rayed, have blood work done (maybe including an EC titer if you are convinced that is the cause), and have her put on Metacam to reduce swelling at the spine/nerve if that is the cause.