Kate your blog is touching and strikes a chord with me. Having 'been there', in a pit of despair at times, and moving on initially and trying to forget about it, there is so much I can relate to. My M finger was the problem too.
My initial solution - simply to keep playing music - was to avoid M and (indeed A as they are inextricably tied) and gently curl MAC into palm, playing with P and I exclusively. It meant re-fingering repertoire, learning new (not too demanding) repertoire and plenty of experimenting (eg playing chords with the tip of I bent inwards and a quick sweep into the strings.) I took my inspiration from Leisner (and corresponded with him) - if it was possible, it was possible, end of story. I played like this for many years including gigging at weddings etc (not recitals though).
Eventually, years later after toying with flamenco for a while (and not really getting far, my symptoms sound worse than yours as they generalised- eg fingers started to curl in and cause problems when doing up buttons) I went back to basics and told myself I would never, ever be satisfied if I didn't get my hand control back.
I stopped tinkering with flamenco and stopped playing with Pand I except for gigs when I had to. I committed myself to a strategy, using every possible spare minute to retrain. I was positive about this and no longer 'feared' the dystonia as I did initially (mindset is very important, I have to say). As I said elsewhere, I spent a lot of time researching how others recovered and started to look for principles - what are the key ideas that move you forward?
Your physiology is fine Kate, your hands and fingers work fine - you prove that by doing other things that also demand fine control. Your hardware is intact, your software is broken.
When windows gets messed up, trying to fix it can make things worse and waste months. Wiping the hard drive and reloading always works. Sure you have to install all your aplications again, and all your settings, and organise evrything the way you like it. It takes a lot of time but its worth it in the end.
When we learn a sensory motor skill we are building a programme, building our software. When complete it runs subconciously which is why you don't have to think about every individual action when you drive car.
FD is like badly corrupted software, a new pathway is written and the more you try and force the movement you want the more the signal is simply sent down the wrong pathway.
There is a threshold at which this happens and you have to be at almost a meditative state of relaxation and awareness, plus of course working at slow mo speeds, to sense it. Get there, open that door just a crack, and you you can start allowing the sensory motor system to rebuild a healthy pathway. Frustration, anger, despair, desire to control the fingers... they all hinder this, you have to allow the fingers to moce freely, which is a different concept.
Have to go now. Shame you are not in the UK, we could have met to to look at this, but I am sure there are people in the US who teach guitar who have cracked it too.