as far as i know there's no way to naturally switch on/off single save mode after starting a campaign, but if it's of any use (almost 2 months later), you can go back to your most recent save by force closing the game with task manager (if it does try to save still, just keep force closing over and over)
i like to also save backups of certain points in the game by going to the save file directory and just copying the file elsewhere, but i'm not 100% sure if there's no sort of failsafe for that sort of thing on honor runs