On the rollback make sure you properly deinstalled the Software and cleared up everything remaining from DP9 first, Remaining Files or Service-Entries can prevent a new Install to succeed or to work propery.
Personally I made a Copy of all relevant DP7 Files first, meaning Config-Files (/etc/opt/omni/server, omnirc) and DB-Files (/var/opt/omni/server). With that you simply install a fresh CellServer-install, copy the Files back and start up where you left off.
Regarding the failing Upgrade there are plenty of possible causes. Most times it is the IDB that has some unclean Entries that make the conversion process go mad. Plenty of those errors are fixed in Patchbunlde b901, definitvely install that first before starting to work in the upgraded Env. New Patchbundle b902 is also out and fixes some more stuff, but it is quite young.
Suggest to read the Upgradeguide 7 -> 8 since it applies to 9 as well. Basically don't do a live upgrade without previously doing a testrun. For us this was cleaning out the Source IDB (Clear, Purge, Fixmpos, etc), exporting it to ASCII, importing that onto a VM Testserver and upgrading there until we ended up with a working IDB.