Finally put her down last Friday (a week ago) and took her to the state lab. Their finding was lymphoma likely caused by ... wait for it ... Mareks! Exactly what I told my vet from the first visit.
I did feel terrible though, that we had been tube feeding her and she had an eye infection and swelling on left side of her head and roof of her mouth. No wonder she fought me so much near the end. And she lost weight despite my efforts to feed her...weighed only 1.25 lb at the end.
Thank you SO Much for your help. It made a huge difference in my confidence. Probably would not have done the necropsy without your encouragement. Now I know my flock is OK since the rest are fully vaccinated.