A long long time ago, when the world was new and this game was still in EA I made a tool to allow for multiple single-player savegames, and multiple characters inside the same savegame (still single-player only).
I haven’t updated it since then, though to the best of my knowledge it still works flawlessly (I use it when testing builds). That said, use at your own risk.
Download link here:
and how-to-use video (series) here:
Once upon a time I planned to do much more with it (hence the name), but frankly I lost interest, and I don’t really have the time any more either. Maybe if I ever get back into singleplayer!
I am mainly making this topic to have something to refer back to, since I still occasionally see people asking how to maintain multiple savegames, or to be able to “play against themselves” in singleplayer.