I am not very familiar with systemd. The answer I seek with this question is:
How is it possible to make systemd react to problems with "dropping the user into an emergency shell of sorts", instead of simply having an unresponsive system?
Example for being explicit: Case of an install of arch linux on a thinkpad, it seems that there is some miscofiguration of the x-org-server, or wayland, or maybe systemd, or lightdm, which instead of producing a error message leaves the user hung up. This means that the user sees the service/startup messages being output to tty1 come to stop, without any having any error message produced (if there is one the glorious systemd-journald ate it??) and no key combination whatsoever can even produced a root shell to enable the user to check for the error and correct it.
Consequenlty the answer how to configure systemd to drop into a emergency shell to avoid a hung up system at any error, on the way to a graphical.target
How such a hung up situation might be like is done for instance in this U&L question "arch-linux-hang-on-reached-target-graphical-interface"