Glad to hear Buffy is still alive and doing well! :hugs
Your plans sound good on keeping her warm. If you use the panel heater or other heat source with her cage, she should do OK by herself but if she seems stressed, put one of her buddies in with her. None of my old hens have ever had...
I've had old hens just as you describe here. They get to the age where they develop heart disease, have breathing issues, become thin, arthritic, no balance,.can't roost and have to sleep on the floor.
Old hens need supplemental heat on cold nights as they truly can freeze to death and or have...