@Inquisition Time! As I commented below, after the mod works, everything is like vanilla (including Ideo DLC). This mod don't change enslaving and converting prisoner system.

Will relations drop if those same captives are enslaved or converted?

@Big Cheese This mod is just active when arrest immedietly. After then, the game would be same with vanilla.

Is there a way for the other faction to know?

Is it where if you keep them arrested for too long then they'll send someone over and you'll have to give them up? And if you do let them go, they'll tell their guys that you arrested them?