Some ideas and recaps glues together:
Kings/Duchesses could have a useful purpose, like getting free stuff when Mountainhome caravans arrive, with free armed guards or whatnot. Maybe receive a small army support...
If the king/duchess dies, the next time liason comes, he notices...
Investigators should try to find the culprit. If one exists, he should be taken away, with aplied negatives to the fort such as no caravans, double prices, the decreased mood of all friends of the culprit, etc. If they don't find a culprit due to player intervention (or goblins), the Liar skill should be useful (but be somewhat high to work). If that doesn't work, have them search the fort, removing beds, deconstructing in-use workshops and walls (supporting walls... major cave-in anyone?) and other vital activity disturbances. If they leave without finding the Noble, negative stuff should happen (no caravans for years, precious stock seizure, worst?)
Should the Investigators be killed, the Inquisition should come after. They must meet with the Leader, then suffer a mass Interogation of all appointed nobles, mayors, etc. making the fort without leaders while decreasing everyone's mood to content max. Then decide to punish a random dwarf and take him away for trial (and never to be seen again in your fort).
Should the inquisitors be killed, full-blown war!