-Having a big druchii kingdom without Malekith should indeed be a near-impossible task. Malekith achieved what he did with Naggaroth through a meticolous control since the very beginning and for thousands of years, during which the druchii became more and more deceitful and ambitious. With him gone, it is unlikely any other elf could manage to replicate the same achievement before the World Ends.
-As follows, there is no way to get the Witch King government if you are not the Witch King.
-I do not know about the magic items, as for power, Ulthuan starts stronger, but is waning, whereas the Empire starts broken, but can rise again. If an Interventionist Phoenix King manages to establish enough stable colonies, those can provide a lot of income and military support, but if Ulthuan remains isolated, it is destined to slowly fade away.
-That depends on the playstyle, I suppose.
-I think that is more of a localisation problem with what concerns river movement.
I know this is now an answer to an old question I asked, but any child of the Witch King will have Witch Kingdom as form of government. I just wonder if it is possible to marry and thus get the form on heritage. In my current game he has a child, I just don't know how to approach the situation, because it was a HUGE change to the Naggaroth situation.