Definite Progress and probable success. First of all I had to re-install the 9.23 driver and this left me with 2 Spacedesk video adapters. I uninstalled the one with the exclamation point and then followed your instructions with my phone connected to my PC with a USB cable. For the first time I saw my PC screen on my Android phone. Although my nVidia control panel did not see the Android phone (this is to be expected), Windows did see my Android phone as a third screen. I then disconnected the USB cable and the phone was still connected on my WiFi network. I was able to move an open window from screen 1 to 2 to 3 (the Android phone) and back. I was also able to open a file with my mouse while a window was open on the phone.
I didn't spend enough time experimenting to comment on the performance (or lag over the WiFi network).
At this point my biggest questions are--
1) a smoother install process (I assume you are working on this)
2) the video settings on the phone are the native settings of the phone and this makes the phone screen hard to use--In my case the OnePlus 7 Pro has a relatively long aspect in landscape mode which distorts the image on the phone. I did not attempt to change anything on my phone. Is there (or will there be) any way to make different Android phone screens easier to use with your software?
3) Will you support wired USB connections or will connections be limited to WiFi?
In the meantime, thanks for you clear troubleshooting instructions.