Loving all of this. The further into the diary I got, the better things got as well. The best (from my perspective) is the matrilineal marriages bit at the end, which I seriously didn't expect after the 1.1 direction shift on the issue compared to when the game was still in development. The Suomenusko split is also unexpected and also very welcome.
I've got some questions though. Given how some of the new UI features are ones returning from CKII (like the kill list), is there any chance for an individual character's family tree? It shows different stuff and serves a different purpose than the dynasty tree, so I don't think it'd be out of place. Though, given the more streamlined UI of CKIII, perhaps it should use the vertical rather than horizontal orientation of the family tree, so it'd have the same layout as the dynastic one.
Also, when you get an inbreeding warning on marriage proposal, could the warning show the common ancestor that triggers it? If there are multiple ones, just the most recent would do.
And while I'm on the topic of trees, the dynasty tree is lacking the filters the CKII one had (if family tree does make a return, it also should have them). Likewise, the find character menu is still lacking some filter options from the previous game.
Finally, the mantling piqued my interest. How moddable will it be? Will it be possible to unlock it for AI? Will we be able to give it conditions, like showing only for titles ruled by someone from a dynasty with high Level of Splendor? Will it be possible to make mantling change depending on conditions like the aforementioned Level of Splendor of the ruler's dynasty or the government type?