ORIGINAL: Noyyau
So I did set up a small test after all, and what I discovered what that, while research from explorers works as intended, the !setdifficulty command does not.
It does indeed use the custom parameters, but about at most a game-month later (sometimes as soon as a week) everything reverts to the original difficulty.
I also saved the game while the custom difficulty was active and reloaded this new savegame: it did start with the custom difficulty active, and about two weeks in it reverted to the original difficulty.
I did check and no, "difficulty scales up" was not selected at game creation.
Well that sucks. There most be some code that reverts the difficulty that I didn't notice. I rarely get the time to actually playtest my changes for very long these days. The next time I look at the code maybe I can find it.