If anyone is still watching... Any luck with figuring out this one?
Couldn't get drivers controlling a rotation to work on that model here at Sketchfab (they did work in Blender), so I used the transforms constraint. However the rotation for the controlled bones is off by a few degrees. (The bone axes have been shifted.) Any idea why that's happening? In Blender they are aligned correctly as they should be.
Also if it's an issue between Euler and quaternion, I tried Euler - but that causes issues with movement snapping. (Doesn't seem to account for going past 180°.) Quaternion fixed that, but I'm wondering if Sketchfab isn't interpreting it correctly.