I've been accompaning this thread for some time, but unfortunately had nothing to add... Until now.
I see defining the de jure setup has generated a lot of different opinions and I'd like to propose a solution. Considering the many different groups that inhabited most regions during this period, it might be a good idea to use a system similar to that present in the Winter King mod, in which, by controlling certain provinces (the CK2 kind, not the roman one) and creating a title, you trigger an event that changes the de jure setup. While this might be somewhat complex it probably would be one of the best ways to portrait the rise and fall of kindoms during the migration period.
Please feel free to say what you think abou this idea, and if there's any interest I might volunteer to try and implement this.