The writers already stated that Ner'Zhul and Arthas were gone for good.Arthas put the helmet on with the Lich King's permission... Unless you are going to try to declare that the Rise of the Lich King novel is non-canon (which would be laughable, since you do not have the power to do so), Ner'zhul did not take over his body. That novel wasn't third party accounts of things that people in universe assume happened, it was a word-of-god account of the events that resulted in Arthas himself becoming the dominant personality of the Lich King. The only thing it isn't clear on is whether or not Arthas actually destroyed Ner'zhul's spirit, Arthas "killed" him in the dream the same way he killed Matthias Lehner, but Matthias was still around for us to find later, which to me says the Ner'zhul just went dormant after Arthas became dominant... With Arthas now dead and the helm passing to a new owner, Ner'zhul could be playing the long game, slowly and subtly corrupting Bolvar as his new host.