General answer to all 3 questions: I think that auditory processing-based theories can't be the whole story. It's got to include problems with "feedforward control", which is our ability to speak a sequence of sounds without relying on feedback. Some neural "glitch" - currently not understood, but probably involving planning-related motor cortex, the basal ganglia, and the cerebellum - occurs between the steps of knowing what you want to say, translating that plan into a sequence of movements, and sending those movements to the muscles that control speech. Auditory feedback certainly has something to do with stuttering, which you can easily see through the fact that, for example, stuttering is dramatically altered by putting in earplugs or using delayed auditory feedback. But I'm not convinced that stutterers' major problem involves auditory processing.