I confess to no specific knowledge here, but I suspect none of the
firewall front-ends will accommodate an arbitrary iptables ruleset, as
the front-ends impose their own structure which would almost certainly
conflict.
Unfortunately, ufw doesn't have a safety net.
However, I did keep a valid ssh connection in a separate window to ensure I could still login after I enabled ufw. That's still a dangerous way but my fallback is my server is with a company who can assist in a reboot and ssh access again if necessary.
Alexander's idea is a good one, and I really should have taken his advice. However, all worked out well, fortunately.
Thanks, Joe.
-Tom