I ended up starting all over again, with a clean install of Ubuntu Mate (available from the main Ubuntu Mate web site). This time I used Rob Lancaster's installation/setup script: github.com/rlancaste/AstroPi3/blob/master/setupUbuntuSBC.sh

There is nothing at all wrong with Damian's walkthrough (and I still needed to redirect gpsd to the correct place). But I noticed that Rob's script includes configuring network manager to put up a hotspot if none of its known networks are available, and it configures the ethernet interface for direct connection to a pc. You could set up Damian's way and then just follow along the commands in the script as well. I have not tried using the 2nd wlan adapter yet, will cross that bridge eventually.

The script installs x11vnc. It works, but is disappointingly slow and clunky. I've been spoiled by VNC Connect on the Pi, but not so spoiled that I want to pay $4/month. I may try another vnc option or just go back to NoMachine.

Mate gives me a layout I am happier with, including a Windows-style quick-switch tabs bar. I also spent a while going through every KStars option panel (I think - I probably still missed some) and learned you can add a Toggle Mount Control to the main KStars tool bar. Which is good, because I still don't have a minimize widget on Mount Control (even using the PC directly with monitor connected). I wonder if that is a recent KStars change. I also added a Toggle Mount Tracking button, for those times when I have balanced a light panel on top of the tube and don't want the mount to rotate.