The simplest way to figure out what file holds a setting is to modify the setting in question and the find the newest file in ~/.config and ~/.local/share .
I don’t know what problems you havd with kscreen; I got my non-standard redolution working just fine by defining it via xrandr. It now shows up in the display kcm and the setting sticks. Even for my AV. amp which doesn’t have a screen attached .
