Removing the old udev rules did indeed resolve the precipitating error and allow a smooth boot.
I kept the files so I could recreate the boot dependency failure, so if anyone has an idea as to what output would be useful for me to collect to diagnose the strange emergency shell behavior, I can tackle that as well. -- To UNSUBSCRIBE, email to debian-bugs-dist-requ...@lists.debian.org with a subject of "unsubscribe". Trouble? Contact listmas...@lists.debian.org