Thank you for taking the time to provide your input on Dominique's plight. We started treatment as suggested, but within a day or so, her leg was completely paralyzed, and the crop became hard again, after we started feeding her just a few teaspoons of egg. We realized it was time to put her down. I found that UC Davis Veterinary Medicine will accept sick (or deceased) chickens from a backyard flock and perform a necropsy for only $25, so we drove her up to Davis and handed her over to them. We got the final report back today. In short:
"The main microscopic findings are the lymphocytic multisystemic inflammation with neurotropism (encephalitis and polyneuritis). Those lesions suggest a lymphoproliferative disease, presumptive Marek’s disease."
We contacted the breeder who confirmed their stock is vaccinated against Marek's in ovo. Of course, vaccination is not 100% effective. I have subsequently read and highly recommend this post:
https://www.backyardchickens.com/articles/the-great-big-giant-mareks-disease-faq.66077/
We'll miss her, as she was a sweet chicken. Happily, the other 4 pullets we received at the same time are all thriving. We'll continue to watch them closely to be sure they don't start showing any symptoms. They are about 20 weeks old now, so we are hoping we'll start to get eggs soon.