As for the issue of holding it when he needs to poop, he's actually created a severe case of chronic constipation and has been medicated for this for years. It's not the exact issue you are dealing with, but I think it has something to do with control and hyper-sensitivity. Did she have a bad potty experience (i.e. painful bowel movement) in the past? This can cause kids to develop a fear of pooping, and she might associate pain or discomfort with the toilet.
This has been our life for two years now, and I do think hypersensitivity has something to do with it.
