If there was a single player only option that disabled drop in multiplayer I think some of these ideas would be more feasible, namely shared gold. Though the time to create that option is probably not worth it.
There already is such a feature. It's the small gem above the minimap; click it, select "Nobody" and uncheck "LAN Connections". That effectively makes the game singleplayer.
I thought you could also choose that right at the beginning of a game? There is a menu before you start a new game where you can choose between singleplayer and multiplayer...
