An update:
Curiously I am able to remove the container from the Docker tab, remove/rename the nzbget folder in my appdata folder, pull down the container and it runs fine. I can stop the container, pull in my old nzbget.conf, then restart it (so I retain my path settings, etc easily) and it continues to work fine. I can then also re-install VideoSort from here (https://github.com/nzbget/VideoSort/releases), version 8.0 at the time of this writing, and it too works fine. I might add that I did install Video sort into the /custom-cont-init.d folder as the previous README advised. As part of the VideoSort install I have also installed Python 2 through the docker's console. It will work fine once NZBGet is reloaded from making changes through the container's web interface as well. If I restart the container altogether, I immediately get the same errors on startup and an inaccessible web UI and new errors as listed below:
s6-rc-compile: fatal: during resolution of bundle user: undefined service name custom-svc-README.txt
s6-rc: fatal: unable to take locks: No such file or directory
s6-linux-init-shutdownd: warning: /run/s6/basedir/scripts/rc.shutdown exited 111
s6-rc-compile: fatal: during resolution of bundle user: undefined service name custom-svc-README.txt
s6-rc: fatal: unable to take locks: No such file or directory
s6-linux-init-shutdownd: warning: /run/s6/basedir/scripts/rc.shutdown exited 111
s6-rc-compile: fatal: during resolution of bundle user: undefined service name custom-svc-README.txt
s6-rc: fatal: unable to take locks: No such file or directory
s6-linux-init-shutdownd: warning: /run/s6/basedir/scripts/rc.shutdown exited 111
s6-rc-compile: fatal: during resolution of bundle user: undefined service name custom-svc-README.txt
[custom-init] removing existing custom services...
[custom-init] no custom services found, skipping...
[custom-init] no custom services found, skipping...
[custom-init] no custom services found, skipping...
[custom-init] no custom services found, skipping...
Still trying to find a permanent fix for this as I naturally don't want to have to do this dance each time I restart the container which will be at least once a week when my appdata backup runs and stops all Docker containers. Seems it does not like VideoSort or Python 2 all of a sudden and I am not aware of a good replacement.