This may not be what you want to hear and maybe it isn't my place to say it, but I think you may have become so fixated on a tiny imperfection that you've lost objectivity here. Your professor said a lot of really positive things here--you did well in the course. An A- is a good grade. Heck, as long as you passed GPAs start not mattering the further in your career you get. It was only when you pushed and pushed that the conversation started getting unproductive.

You can be a therapist. You are meant to be in graduate school. You are doing well. I think being sent to the disciplinary board is a little absurd, but don't freak out about it yet. It's a place to calmly state your grievances and settle the issue as to whether a doc's note excuses the late grade. Worse case scenario, they tell you you were rude, you apologize and move on with life. Good luck.