As an aside, for anyone else who is trying to do the same (and to give some background into my procedures so far), I have been following these references:
<https://docs.mailman3.org/en/latest/install/virtualenv.html#setup-virtualenv>
My goal is to create the least change to file locations and configuration files as possible (I may move things into more "source-installed centric" locations once I have things working again), thus, many files are still in the original Debian package-allocated directories.
That said, I was forced to move mailman3web.db to /opt/mailman/web as sqlite (and, hence, mailman-web) simply could not open the database as user mailman from my virtual environment (even with mailman3web.db file permissions set to 777 and owner.group set to mailman.mailman).
So far I've gotten mailman running properly and handling the mailing lists, however, I have not gotten the web interface working again.