
I was able to correct some of this in the few days between my discovery and the concert, and now I have been practicing with a specific focus on keeping my left-hand thumb in the same relaxed position relative to the rest of my hand no matter where I am on the instrument.
I realized that it is exactly the same concept that I used to use in flute playing: the idea of a flute emboucher that works for every single note on the instrument without having to adjust the angle or the lips or create tension in the facial muscles. Having a flute emboucher that works in such a way requires a lot of development of other muscles in the body, particularly the supporting diaphragm muscles, the tongue, the facial muscles, and the muscles that keep the throat open.
Developing a left hand position on the violin that works like an ideal flute emboucher also requires development of supporting muscles, especially the muscles in the left arm that supports the fiddle. These muscles come into play with me in particular because I don't use a shoulder rest. This fraction of an inch difference in my left hand has changed everything in my playing, even the way my right hand operates. For some reason it seems to have to do less work because my left hand no longer (most of the time) has the useless and energy-consuming task of changing when I move across the strings. Only my bow needs to move from string to string, and it even seems to move more efficiently for some reason. I can now play three notes chords more easily, and shifting from one position to another is a more efficient motion, but one that I need to practice in this new and more efficient way in order for the shifts to be accurate.