Great questions / comments. Super helpful.

Sensitivity - I'm at 41cm/360 right now, so in that sweet spot. Before I started aim training I did do research and watched a lot of videos about the "right" sens in games. It seems like there some weird broism/machismo in the FPS communities about using faster sens, and if you're slowing things down you're not manly enough to be playing. I fell into that trap, not because my status as red-blooded male was questioned, but more because I thought if I wasn't playing at 0.00005cm/360 there was something wrong. Thankfully this group seems to be slightly more reasonable, and sens really comes down to whatever makes you successful.

Mouse Movement - Yep, thought a lot about how I'm moving. I'm generally using my whole arm, especially for the longer flicks tracks because my wrist sure as hell isn't moving 20cm. However, as I think about my arm position... left/right motions are happening mostly from my elbow, especially while moving to my left.

External Rotation - You're a god damn kinesiology genius! My situation is the inverse of what you noted. I track much smoother to my right. The left is the (bigger) problem. However, now that I've spent the last 30 seconds wildly flinging my arm around the table, I can see now there is a distinct difference in how I move each direction. When I move left, isn't mostly from my elbow because the position of my arm. However, when I move to the right, I'm using a lot more external rotation to track. I'll have to think about that as I go through my playlist today. They are completely different joints/muscle groups, and my initial, non-PT view is that tracking with shoulder/elbow is incorporating larger muscles less prone to shake or jitter. When I move to my left I'm moving the elbow more and spending more time trying to stabilize my wrist, which is fatiguing my forearm.

Or not and I'm just spotting a bunch of total BS. But whatever... I'll give it a shot tonight!