Using 'dpkg-maintscript-helper supports mv_conffile' guards introduces unreliability into upgrades; it means that the conffile is removed or not depending on whether dpkg happens to be unpacked before lubuntu-default-settings. This seems generally undesirable; it would be better to enforce a single code path. This matters because Ubuntu's last LTS release didn't have a sufficient version of dpkg for that; I know Lubuntu wasn't in the last LTS, but this is the last package on my list and it would be good to get the list down to zero!
It would be nice to just use dh_installdeb's support for generating dpkg-maintscript-helper commands, which was introduced in debhelper 8.1.0. This would remove duplicate code from your maintainer scripts - in fact, you could remove your handwritten preinst entirely. Here's a patch. (I'd have submitted this as a merge proposal, but as I understand it it isn't currently possible to file merge proposals against branches in the +junk namespace; maybe lubuntu-default-settings should become a proper project and have the packaging branch moved there?)
Thanks for the patch, I committed it to the branch