Here is how I am currently tapping into the three data lines. Once the code is done, I will remove the socket and put the nano back inside the radio. I don't want to bust the encoder plugging in the nano. I made a fingerboard and glued it down so I wasn't tugging on the fine wires going to the traces. I don't yet have UV resin, just UV mask, and that stuff is not strong so I superglued the fingerboard on..

This pin circled in red is the 5v source for the arduino. I made my fingerboard one finger too short.

The only bad part is the need to invert the CS signal, so I currently have a NPN transistor inverting it before going to D10. Haven't figured out how to flip that in the code yet.
And here are the pinouts labeled. Dont forget that I have an external inverter flipping the signal from the radios CS (labeled D10 on the radio pic) before going into the arduino's D10 input pin.

And in case I never get back to it tomorrow, here is the code as it is now, with the channel tables. Instructions are commented out in the sketch.
Important... When you write to the arduino, you don't want radio power to it. If the radio is off, it pulls down the arduino voltage. If the radio is on, there is a small voltage difference. Just unplug the arduino power lead when programming via USB.
edit to add some notes in the arduino code