Thanks, carmar.
OK, I have made a little progress, but not much. Continued reading other solutions buried in your last linked post. After getting to the shell, tried:
sudo apt-get install --reinstall lightdm
Nothing. There is a suggestion to purge lightdm, but it looked as though it might take Zorin down with it. Also a suggestion to reinstall xserver-org, but don’t know what that is.
My understanding is that lightdm is the manager by which the user logs into the desktop. Is that correct? There’s an additional suggestion to try installing a different manager, gdm. Would that be worth a shot?
Finally, from the shell, I tried:
startx
That gets me into the desktop. But I can’t figure how to get there directly by booting into it. Any thoughts?