I think it's still too passive: no matter if you're the liege of the diarch, if you're both well-liked in the realm, nothing really happens, and the player can't really do anything to spice things up. Hence my suggestion on some extra monthly swings. Apply to both liege and diarch.

* Friend with powerful vassal or powerful house head increase swing.
* Having strong hook or imprisoning them also increase swing.
* Friend relation between liege and diarch slows down swing in either direction
* Diarch/liege having strong hook or imprisoning the other will greatly increase swing and removes maximum and minimum threshold.

The idea is to give players the sense of agency in permanent regencies that by being proactive with the swings, it's eventually possible to coup a popular liege, or to defang a powerful regent.