So if you are a Duke without region and your ruler refuses to dissolve your Duchy, you cannot have an estate or a Lordship, ever?
Nope. An estate assumes an oath of service to the lord of the region. You become that lord's vassal, and they become your liege lord. If you are above them in station, then how can you take an oath of service with that lord?
You can always step down from the duchy. Then you won't be a duke, and you can then take service with another duke. Then *he* can give you a new region.
The way it works now makes more sense. If you are above someone in station, or at their level, then you can't take service under them. It makes absolutely no sense at all for the ruler to have an estate in a region, owing allegiance to another lord. The ruler is that lord's ultimate liege. Why the heck would the ruler be swearing an oath as that lord's vassal, and paying them taxes, then taking his own ruler share from that lord's duke?
The old system was horrible in that regard, but it's the best that could be done. Now it should be much simpler, and clearer, once you you discard the way it used to work, and take a fresh look at the system.