I have a current game – started under version 2.2 and upgraded through 2.4.2 – where I'm Emperor of Britannia, King of Scotland, Ireland and England and have vassal King of Wales. The inheritance laws must indeed be changed separately for Britannia, Scotland, Ireland and England – all of them are now Primogeniture, but I think I had to change Scotland from some form of elective when I usurped the title. I don't have control over the inheritance laws in Wales.
I have two duchies, both in de jure England, one of which contains my capital (I moved to Middlesex/London, after I finally squashed Usurped the English). These both follow the Primogeniture law, which doesn't really help us determine whether they follow the Kingdom or the Empire's laws.
I do have one county in both Wales and Scotland, so I can quickly test the succession laws for counties under titles you do and do not control. The Wales county may or may not have de jure drifted into England.
I also created two Kingdoms in Africa for the Prestige and was depressed to see that these were created with Agnatic-Cognatic Gavelkind succession.
I will do a series of experiments and report the succession laws that apply:
- Create or revoke a duchy under a Kingdom I control other than England, my primary kingdom.
- Create or revoke a duchy under a Kingdom I don't control, but belongs to a vassal.
- Experiment: Kingdom of Castille, existing and held by my direct vassal with current succession law AC-Elective. Created the Duchy of Castille, de jure part of this Kingdom and it recieved AC-Primogeniture, my succession Law. It is de-facto part of my empire, not the Kingdom of Castile.
I can't change my succession law, because some vassals are fighting each other, so I might have to create several more Empires to get them in line.
- Create, revoke or usurp a Duchy under a Kingdom that doesn't exist. (This will probably be under the Holy Roman Empire, which has a lot of duchies and no kingdoms at this point. It probably won't follow the HRE succession laws.)
- Get a duchy under a Kingdom that is completely outside my Empire. (I'll probably have to grant independence to an African kingdom to pull this off. Getting a Duchy under an independent kingdom is probably going to be slow and painful.)
- I have briefly held duchies that were holy-warred from Muslim Kings who were under Open succession, which can't be used by Catholic rulers. These defaulted to Primogeniture, the succession law of my Empire at the time. It may or may not work differently.
I fortunately have Imperial Administration, so I should be able to revoke duchies and heal up the rifts reasonably well enough to meet the 'no vassals have negative opinion of you' to test succession law changes. If anyone can think of other scenarios that I can pull off without too much more trouble I'll be glad to add them.