This topic is a great read! Although I cannot submit any useful code, I did some thinking. Here are some thoughts.
One thing that keeps coming back to me is that the velocity control will probably need an exponential curve (rotation speed vs. increments). This is more based on say "professional intuition" than fact. Though there are so many things in nature that are modeled as first or second order exponential curves that it seems highly probable. It is probably explained in that you want precision as you turn slowly, but want speed as you turn quickly. As mentioned before this should feel as a smooth transition.
Another thought was this: does a user really want/like velocity control? If a user wants it, what do we need to do to make it feel right?
I recall working with the BK precision electronic load and being not used to the velocity control I turned the rotary knob big time to increase the load current (knob was set to the least-significant digit, or so I thought). Due to the velocity control with that knob, the current was at max within what felt like half a turn, though probably in reality was 1,5 turn or something. It didn't feel right. It took me a long time to get used to the control having non-constant increment depending on rotation speed. I felt I needed to be extra careful while adjusting the load with the output on, not (yet) trusting the instrument would do what I wanted.
So what think is that you need some clear feedback to the user that is more than just the numbers on the display increasing. The change in the displayed value is probably not really intuitive if you're used to look at a certain digit changing, or looking for a constant change, as you rotate the control knob. Maybe if velocity control kicks in you should indicate in real time which digit is increased so you aren't 'overwhelmed' by the way your value changes. (Is it clear what I mean here?)
P.S. read for increasing also decreasing, but i.m.o. that's just a matter of rotation direction.