I got the project working, but not without some experimentation. Remember my ground question back in post #10? Something seemed odd to me about using ONLY the positive output from the high voltage source. None of the posts or schematics I could find on the web seemed to say anything about the high voltage source's ground connection. When I started working with the MOSFET, nothing worked. Then, I connected the high voltage source's ground to the UNO's ground, and everything worked. Noobie error!!! Thanks for all of your comments.
P.S. I'm waiting for a delivery of digipots so I can try that route, too -- just to learn about them.