After 3 months of going Little Red Hen on this, I think I figured it out. My system had live-config & live-boot (with their dependencies, live-boot-initramfs-tools, user-setup, live-tools and live-config-systemd) installed. An aptitude purge command on these packages got my system booting normally again.
Despite http://debian-live.alioth.debian.org/live-manual/stable/manual/html/live-manual.en.html#139 saying that installing live-config & live-boot 'will do no harm and is useful for reference purposes', it clearly bugged up booting. I can't say that I'll miss having to hit E at the grub boot loader, add single or systemd.debug-shell to the boot parameters, F10 and type `sudo systemctl isolate default` at my first shell EVERY SINGLE TIME I boot my computer. I only hope that others benefit from my suffering and the irreplaceable hours of my life sucked into this time vampire so that the cycle of despair can shorten if ever so slightly. Off to whine to the package managers unless there are any insights to explain how these packages can coexist on my system.
