I figured it out. I needed this to tip me off: dpkg --get-selections|cut -f 1|xargs sudo apt-get --reinstall install -ud 2>&1 \ |grep 'it cannot be downlo

I figured it out. I needed this to tip me off: dpkg --get-selections|cut -f 1|xargs sudo apt-get --reinstall install -ud 2>&1 \ |grep 'it cannot be downloaded'|awk '{print $3}' Some packages were not being downgraded because apt-get won't downgrade them without explicit directions on what version to download, even if it's the only version in pool for your target. I needed either libalgorithm-diff-xs-perl, libpthread-stubs0, or libtext-charwidth-perl, and after that, only menu was the last remaining Segmentation fault. Extracting the package manually and replacing /usr/bin/update-menus enabled me to reinstall the remaining packages without errors from debconf and friends. -- Sorry, spoiler, if anyone was following my awful example. I definitely did not follow the shortest path thru this maze.