I still believe that Ner'zhul's soul is still in the crown. It works like Kel'Thuzad's phylactery, or any lich for that matter. Ner'zhul dies when the crown is destroyed, but cannot be because the remaining scourge will go bonkers. After Bolvar is crowned, it sounds like an orc is telling Tirion to get lost.
As far as Arthas, Frostmourne, and Ner'zhul goes, it was Ner'zhul and Frostmourne/Arthas that merged together. Frostmourne had taken most of Arthas' soul and Arthas used whatever he had left to hold back the scourge. My friends always tell me that Arthas was bad. I will never agree with them in a million years. It's unfortunate that the story is mixed up between WC, WoW, and Rise of the Lich King, but I think it makes the most sense that Arthas was good.