Well, I'm not sure "going back" like that is even possible. If it is, it could be risky for the future of the playthrough. Having a quest conclude, save, revert, restart, and save again...that's potentially tempting a corruption issue.
I'll move this to TW3 Mods and see if anyone there knows the correct quest flags to edit. (If it's even possible to do what you want.) A better alternative may be to simply change the result, give yourself the correct rewards, etc. and carry on.
Just be sure you back up all of your saved games before making any such changes.