Well, I certainly agree with you -- it doesn't matter what you do with your breathing. You already know how to breathe, working on breathing will not change your stuttering frequency or severity. Sounds like you got therapy from an SLP that didn't know what to do, sorry for that. I also agree with you that there is nothing physiologically or structurally wrong with your vocal folds. I would even extend that to your vocal tract (mouth, throat, nasal cavity), your lungs and by-and-large your brain, as well. I have thoughts about the logic of your "stuttering is mental" argument. Should you wish to hear them i'd be happy to oblige.