It wouldn't hurt to treat for mites. Some people become allergic and break out when they handle pigs with mites.
What kind of bedding and what size cage do you have? Poor circulation due to lack of exercise may be a factor.
This page may help you
Guinea Lynx :: Cavy Feet and Foot Problems
Bumblefoot isn't necessarily open wounds. It can be a swollen foot pad or infection.
You may want to switch to towels/fleece and see if that helps. May be softer on her feet. Sometimes a longer course of ABs is needed.