Requirement to be lord: be elected lord, even if you have an estate in another region. OR, if appointed, the region lord must not have an estate in a different region.
Requirement to be duke: be a lord of a region in the duchy, or not have an estate. Granted automatically to lords of cities, thanks to old code.
Elections are not yet updated to recognize the new system, they will soon be in line.
Dukeship is NOT automatically granted to lords of cities. That was a ONE TIME THING for the transition only. Basically, old dukes remained dukes, we just split their titles. No need to even mention that on the wiki, as it has no consequences for the future.
So if you remove these two artifacts, it is all really simple.