There are a few things to be aware of.
- what recovery did you use for your backup of the stock ROM? CWM or TWRP, since they are not compatible. If you used CWM you will have to go back to CWM first to restore the backup.
- for the stock 4.0.4 ROM the base directory for the recovery is /data/media/TWRP(CWM) - after migrating to multiuser in 4.2+ it is /data/media/0/TWRP(CWM), so the files simply can not be found by the recovery. So if you use CWM make sure the files needed are in /data/media/CWM.
- on the top-level of TWRP you have options for backup and restore, these are for Nandroid-Backup.
So if you take a backup of your current 5.1 ROM with the recent TWRP 2.8.x experimental, you can restore it afterwards if you either stay on this recovery, or update it first when comming back from the stock ROM.
For restoring the stock ROM it all depends on wether you can access your backup-files. If you manage to get access to them by either using the appropriate recovery and/or the matching file-paths - just do it. If not you will be forced to use odin anyway...
Regards
Thanks a million for this. I think I know what I need to do now. Is there a version of CWM that is flashable from TWRP - or do I have to go with the ODIN one?