Speech and language therapist here. Anything I say is just my opinion. EDMR therapy and making sure your wellbeing is taken care of can be good ways to help manage a stammer as stammering can have psychological links I believe that is no evidence that talking to yourself affects your stammer except that some people find they don’t stammer when they are alone. This could be due to less anxiety which means less pressure to speak. For the breathing in through your mouth for vowels - this has little to do with the fact that they are vowels and more to do if the person is a mouth breather or if they are displaying compensatory movements when they breath (e.g. shoulder elevation and rising of the chest are seen as compensatory movements). Abdominal grading is brilliant for expanding oral statement length in speech whereas things such as rising of the chest to take a breath can be seen as a compensatory movement for airflow quality