On a related note, if I am a Ruler and a Duke, what will happen if I click the "take estate" button in a lordless region of another (since my duchy is empty) duchy of my realm?
I can actually answer that, having done it myself: You cannot be simultaneously under and over someone in the hierarchy. Therefore, even if the land has no lord, it still belongs to the other Duchy and now the other Duke breaks your chain. You will be forced to abandon the estate on the turn change, if everything is working correctly.
Similarly, making yourself lord of that region (through elections, appointment, or other action) will have the same result, also removing you from the lordship.
There's no real harm in testing it, though, just to be sure it's still working properly.
