I had a hen with egg yolk peritonitis (EYP) who lived for well over a year with the condition. If a bad infection does not settle in, than they may be able to survive with the condition. I suppose it does become more and more uncomfortable for the hen as it continues, but the hen may produce fewer misdirected eggs as she gets older, so maybe the belly wont grow as fast. My hen lost the feathers on her belly like your Penny has, and she could waddle around okay. Unfortunately, the other hens in the coop were tempted to peck the bare skin on her belly, so I had to bring her in to live in a large dog kennel. I would bring her outside for supervised visits so she could sand bathe and peck around. She kept her appetite up to the end when she finally sucumbed to an infection.