So, my 1st attempt to fix the Tanistry problem was to edit the succession law, so that it would allow me and all my vassals to use it without the admin law. When I loaded the game, it still dropped to Gavelkind, even though it shows Tanistry to be valid. (I could even console to Tanistry, but only on my primary title.) Apparently the conversion from 2.5.1.1 to 2.5.2.0 save file simply doesn't care. So I saved again, then went in and edited the save so that everybody who had Tanistry before got it again. Loading that file, it appears to have worked.
Additionally, before I saved that file, the laws weren't showing up correctly. There was only 1 under Realm Laws, and none under Obligations & Council. Now they all showed up.
So, I'm going to resume playing this campaign. Hopefully nothing else game-breaking shows up. All ly realm laws dropped to minimum, and all the tax/levy laws went to center, so I guess the conversion from pre-conclave laws to conclave just sucks.
Additionally, before I saved that file, the laws weren't showing up correctly. There was only 1 under Realm Laws, and none under Obligations & Council. Now they all showed up.
So, I'm going to resume playing this campaign. Hopefully nothing else game-breaking shows up. All ly realm laws dropped to minimum, and all the tax/levy laws went to center, so I guess the conversion from pre-conclave laws to conclave just sucks.