I’ve forgotten why you didn’t use her when Becky was out? Was she lame then too? Navicular is something that can be handled with proper shoeing and no hard work - at least for a long time. Of course, arthritis can be too, and exercise is generally helpful there.
A week before Becky came out Tiff said she was running in the pasture, slipped and fell, and she thought, hurt her shoulder. She was better by the time we needed to make a decision, but we decided Mia was a better choice to be sure.