It does sound like a reproductive issue if the abdomen is bloated and firm. When it's very firm that can mean infectious material, when it's water balloon like it's fluid (ascites), from the liver leaking. Those can cause slow or stopped digestion, which show up as crop issues many times, or...
I don't think it's Marek's from what I can see. Do you have Marek's in your flock, or do you only think that because of the legs? At her age a reproductive problem is possible, or other cancer, or organ failure, those are not uncommon. Depending on what it is it can cause all kinds of...