You can switch between all the seats while still in single player? That should be enough? Not that I know anything about modding other than coding.
I've never seen an option for that? Will have to check again, if it is possible, then I can test it that way for sure.
Should be Control+G by default.
I should elaborate. You need to create game in multiplayer, off of a single player game. Once you've loaded in, you can switch between all seats with control G, without having someone in the game with you.
