I wanted to provide follow-up on this in case anybody else runs into it in the future.
The error messages I was seeing during the install of "sssd-common" are
apparently typical/expected and did *not* have anything to do with the problem here.
Alexander was crucial in saving me a lot of time digging around. :-)
The wound was self-inflicted, but not purposefully, and I think there may be a bug in a
Debian/Ubuntu package that was contributing to it.
As part of trying to get this working while also trying to document my steps, I believe I
may have uninstalled the freeipa-client package, and then used `apt autoremove` to get rid
of any unneeded dependencies (which includes the package libnss-sss, which is what
actually modifies /etc/nsswitch.conf on Ubuntu).
When libnss-sss is installed initially, it makes changes to /etc/nsswitch.conf. When it is
*removed*, it undoes those changes, which is actually not normal/expected behavior
(typically, Debian/Ubuntu packages leave their config in-place unless you *purge* the
package). If you subsequently re-install libnss-sss, it does *not* modify
/etc/nsswitch.conf again, unless libnss-sss has first been purged.
So, through install and then remove (via "apt autoremove") and then subsequent
reinstall of dependencies, I left libnss-sss in a state where it wasn't modifying
/etc/nsswitch.conf to add the "sss" entries required for FreeIPA to actually
integrate with everything. freeipa-client itself was working correctly.
I feel like this may be a bug with either the postinstall or postrm scripts for this
particular library package, so I'm discussing that with Ubuntu. :)
Thank you again to Alexander for the help and to everyone involved in IPA/FreeIPA for all
of their work on what I'm finding to be a pretty powerful system.