I've been experimenting with the pronation/supination in RH posture (in part encouraged by one of guit-box's observations) and in some applications a supinated position seems to minimize the focal-dystonia-like issues in my m finger. The approach is promising enough that I've reshaped my nails to better accommodate the position in order to give it a fair shot. I have some obvious speculations about why this works but they're preliminary and, besides, likely specific to my particular case of FD.
I'm also having some success with practicing very quietly and softly. I find that taking volume out of the equation lets me focus better on things like identifying and eliminating tension and optimizing nail shape; I expect it will eventually improve my volume by letting me re-examine my approach to dynamics from the ground up. I've realized that there's no end to the process of identifying and eliminating tension. I'm also enjoying the immediate benefit of not having to worry about annoying my neighbors, even late at night, as well as a much sweeter tone, which makes practice more pleasant, and which I'm taking as indication that there's much room for improvement in my approach to both dynamics and tone.
I've also noticed the chapter near the end of Kappel's "Bible" that goes into the nuances of fingerings, including optimal RH finger sequences for various types of fast passages. Lots of food for thought and practice there.
In all this, I've discovered that it's easier for me to tremolo with p-i-m-a than with the typically taught p-a-m-i. it's something I've never heard or read about, and I'm curious if that's a rare thing.
Regarding focal dystonia, it seems to me that no one has ascertained what causes FD generally and how to get rid of it, and it's possible that the symptoms can arise from a variety of issues. In other words, while any individual's focal dystonia may resemble another's based on symptoms alone, that doesn't mean that the causes are the same or that what helps one person will help another. By the same token, the apparent cause or relief of one person's FD can't be dismissed just because it's inconsistent with or contradicts another individual's experience. It's an unfortunate and frustrating state of affairs for those of us afflicted, but I think we'd do well to keep it in mind when we discuss FD.
What I appreciate most about this thread, more than any particular observation about finger movement, is the skepticism and empiricism that informs the project. While my RH issues made me open to experimentation, this thread helped free me from an unhealthy deference to received wisdom that I was not able to shake on my own. (To my fellow participants who seem quick to perceive skepticism as disrespect, and deference as respect: I assure you that I use these terms advisedly.)
[Edited for typos, grammar.]