Single Save already is an option. It's literally the first option in Customized difficulty whereby you can then select the difficulty between Explorer, Balanced and Tactician (However, this thread was created long ago, before Custom difficulty was added to the game)
As far as toggling mid-run. There seems to be technical reasons regarding that as the setting specifically mentions the setting cannot be changed during the run.
In regards to continuing an Honour Mode run in Honour Mode... They'd first have to make Honour Mode settings available in Custom Mode and then likely make an option to continue with Honour Mode difficulty post defeat (As currently it defaults to Tactician, because Honour Mode is acting as a Challenge Run whereby it's a run where you beat the game without your PC dying)
