The previous cumulative file is just part of the snapshot information we persist in the history. For one-offs it points to the current cumulative patch-id, for CPs to the previous. We might want to use that for consistency checks or perhaps just remove the redundant information in case we don't need it in the end.
Yes, we are going to backup the patch.xml and misc contents as part of the patches/patch-id (then it could also get re-applied). Since we have the rollback patch.xml we should have all the information we need to detect the points in history where a content item was overriden.
Regarding rollbacks i am less worried about the contents, rather the configuration backups. Perhaps we should provide 2 different operations - one for rolling back to YY, which would also rollback XX and a "reset" operation which will also restore the configuration.