Whatever you do, Backup everything first.
That said, and if everything is on one partition, why not boot into 9.1 and delete the OS X System folder? This won't get rid of all the X stuff but it's a start. You can continue to backup and delete files a few at a time to see if it's OK to trash 'em.
Or, Copy all the 9.1 control panels, extensions, prefs, etc to a backup disk, reformat the drive and re-install a clean 9.1. Then add all your old files back.
However! I believe OS X does something to your 9.1 system. I'd check with people more knowledgeable than I am before doing this...