Just reading this now , but I have some similar symptoms in my year old barred rock hen. She started walking upright with tail down a couple months ago. I consider being egg bound and water belly but doesn't seem to be those. She was still laying well and actually turned broody a couple times. She is slow, will occasionally wing drop and doesn't seems to tolerate the heat well. She seems to pant and lift her wings to cool more than her flock mates. Fecal float ruled out parasites. Vet did not examine but thought maybe an injury. I would think if it was Mareks she would have died long ago as this has been at least two months and no others are affected. Maybe vitamin B deficiency?

and rip for your sweet girls!