I love your Nagash conversion so far, but I'm not as convinced by your game interpretation. What if the throne is killed, but the vampire isn't? So nagash dies, and his corpse poops out an infantry size vampire lord? What if the vampire is killed? Nagash leaves the swarm behind, sure, but then there's still a couple of non-character vampires riding that thing that don't seem represented in any way.
Caster lord on throne is probably a better representation of Arkhan than Nagash. Iirc the lich was part of Nagash's war on the empire as well. The big guy would probably be better served by just a 40mm base, with some skeletons crawling up out of the ground, and run him as Count Mannfred + unit filler.