Musicians with Dystonia Bulletin Board

Whether you end up calling it FD or not, you have muscles firing out of control. This can be very difficult to change, but luckily there are more and more people documenting ways they found freedom of movement. Unfortunately there's not yet a simple prescription, mostly because the specific symptoms can vary so drastically from case to case. Some people have apparently found a general "cure" or enlightenment where the brain got fixed and symptoms went away, but I've never talked to one. I have talked to quite a few people who, like me, found retraining strategies that addressed the malfunctions and got some or even full recovery.
First, as daved said, you need to take off the pressure. Continuing to try to play, let alone perform, by fighting and working around the problem will most likely make things worse. Try to imagine only doing movements that have no hint of "weirdness" for the next couple months.

I'm going to step out on a limb and offer my thoughts on what I see in your video. . I cannot promise anything I say will help.
And I suggest that taking anything anyone says and doing too much of it will make things worse.

In my view, it's hard because you need to relearn something you first learned in the womb. It's not what you learned in your first guitar lesson, and "starting over" with even the best "technique" isn't likely to fix things. Even the idea of "un-learning is harder than learning" doesn't cut it for me. Things need to get re-wired at a deeper level.
The nastiest element is that the primary tool to influence this is the same thing that broke things: repeated movements.

You should read up on all the documented cases you can find of people who had success retraining, and see what feels good to you. Take everything with a big grain of salt and accept that ultimately you'll need to find a new way to *feel* your hand, and other people can only offer strategies and tactics.
A new self-awareness and perception of movement needs to evolve.
All the generalized ideas of getting in touch with bigger muscles, relaxing your mind, releasing tension throughout your body are *good* and important things, and something may be the key to your recovery. But meanwhile I'm going to probe some details with specific questions, and you can let me know if it seems to lead anywhere.

Sometimes relief can be found in indirect ways. Let's start by trying to ignore the thumb and try to leave it perpetually relaxed, and get back to the pinkie daved noticed. Were you already aware of it kinking up when the other fingers move?
What would happen if that finger could become and remain a wet noodle, just hanging relaxed while you play?
That can only be a good thing. Sure, a lot of players have pinkies flying around and it doesn't harm their playing.
But learning to feel and sense that finger too can be a healthy part of the process.

With the hand completely hanging loose (no attempt to curl fingers into a "playing posture"), just play one reststroke with "i". Does the pinkie (I'll call it "q") curl up or tense up? Wait ten seconds or more and let all muscle tension relax out of i, and see if anything changes in q (or that side of the hand). If it's as limp as a sleeping baby, try repeating i ever so slowly but decreasing the delay between notes, and see what happens. If q is silent, do the same thing with m and a. The last step is to alternate i/m.
At some point, maybe even with that first note, you'll feel the weirdness. If so, stop there. Notice whether you feel it in the thumb at the same time as q. Wherever that starts, that's your threshold.
Stay on the "freedom of movement" side of the threshold and don't let the undesired flexing happen. Start discoverying what things affect or trigger the malfunction. Notice your posture, your neck from leaning over to watch, the arm movement as you try different strings. Close your eyes and feel it without seeing. Perhaps reach your left hand under the guitar neck and around to touch the right hand. Does touching different spots affect things? If q rests ever so lightly against the pad of your LH fingers, does that make it easier to feel when it tightens?
In particular: If you lightly hold the tip joint of q fully extended by placing the LH thumb against the back side of that joint, and the LH fingers against the palmar side of q, what do you feel? Can you play "i" without q's tip joint flexing? Does holding it straight (even applying some light squeeze from the LH, but carefully and only for a few seconds) change anything?

One second observation before I stop writing so I don't break the internet: Our hand can do a "cupping" motion by bringing the base of the thumb towards the base of the little finger. This actually curves the plane of the hand itself. Watch your palm and do this a few times, and get a feel for what it's like to completely relax it out (maybe doing a light stretch back, using the LH) and then squeeze again. Sense if you can squeeze as little as possible and quickly recover and let the hand flatten out completely. Watch the BACK of the hand and see the effect on the line of knuckles.
Once you know what it feels like to squeeze those muscles, try to never squeeze them again.
Yes, that's a joke, but clearly any hint of those "opposer" muscles firing will take the hand out of whack. Try playing the slow notes with this consciousness, and see if the base of "q" (that muscle pad is the hypothenar eminence, the big pad at the base of the thumb is the thenar eminence) ever tugs at all towards the thumb.
I mention this because sometimes flexion in these muscles does occur, and we don't even know it. The pinkie movements might be a reaction to that (or not; check it out and see what you feel).
This might be the start of a process that can build your perceptions and lead to a strategy that can help relax q. This in turn might gain insight into fixing the thumb issue. The overall plan is to find what works OK, add parts to find what breaks, stay on the good side, and find a new slow relaxed way to add those parts back in.

This is the first time I've posted this detailed a msg on this site. I'd be interested in feedback from anyone else with the time to read this far as to whether it fits their reality, or if you think I'm crazy. Trying to communicate this kind of thing in narrative is very dangerous as it's easy to misinterpret things. But though there's a lot of terrific info that's been posted here, there are many things that were part of my recovery that I don't see other people saying. If it seems helpful, I'm happy to do more.