I have no problem whatsoever with Diane Duane using hyponeutronium as she was creating something new, not retconing another writers work. Also, Christopher is certanly free to use her creation as long as it's in the spirit that she intended. In The Doomsday Machine, however, Spinrad used neutronium so that's what should be accepted. Expamding the universe = good. Rewriting another persons work = not good.
SPOCK: Sensors show the object's hull is solid neutronium. A single ship cannot combat it.
SPOCK: Negative, Captain. Its hull is pure neutronium. There is no known way of blasting through it.
SPOCK: The object's neutronium hull makes sensor readings of the inner mechanism impossible.
Unless Spock is going to be portrayed as unreliable then the orignal work should be taken as it exists.