I have finally updated the mod, which I have been refrained from doing since the Charlemagne release (they were very busy times for me). I have updated the first page of the thread accordingly. As usual, the mod is also available in the Steam Workshop.
I have been playing around with the new option to change the de jure affiliation of counties in the history files. I have found that this is an excellent solution to the problem of "floating titular duchies", which has been bothering me and others. As you can see by reading the first page, the mod also features (slightly) different de jure duchies maps across different start dates.
Unfortunately after some hard work I have found, by testing the mod, that the feature in question is bugged. In fact, the de jure history of counties is reset in savegames to the structure defined by the landed_titles file. This is by all means a bug, as it also affects Vanilla (try to play Powys in the 769 start date, save, and reload; then check the de jure maps).
I am going to post a bug report soon. For the moment I keep the changes as they are, because I do not want to roll them back and I hope that the problem will be fixed at some point relatively soon. The 769 start date is perfectly safe to play, as its de jure duchies setup reflects the landed_titles file.