Just a follow-up for anyone that encounters a similar problem...
I got the display manager to work again after searching for "Lightdm fails to start" and found this link from the Manjaro forums:
https://archived.forum.manjaro.org/t/solved-lightdm-fails-to-start/27058
and this Ubuntu page on LightDM:
https://wiki.ubuntu.com/LightDM
At the command prompt I ran:
sudo lightdm –-test-mode --debug
and saw that the LightDM "greeter" wasn't being found (the apt-get upgrade seemed to have changed some config).
From the "Changing the Greeter" section of the Ubuntu page I saw that my
/usr/share/xgreeters
directory contained two files:
lightdm-greeter.desktop (a link to /etc/alternatives/lightdm-greeter) and
lightdm-gtk-greeter.desktop
From the Ubuntu page I created a file /etc/lightdm/lighdm.conf (with rw privileges for root and world-readable mode; e.g., -rw-r--r-- mode)
which consists of only:
# lightdm.conf - I put in some comments about it being my hack-work
# (another example comment line)
[Seat:*]
greeter-session=lightdm-greeter
I then rebooted and the graphical interface is working again. ^_^
