I think that PAR, CSO 681 dated 6th January 1879 and signed by Durnford himself (as an official document) is sufficient proof of an official name change having happened. It is certainly not 'conjecture'.
My point is that the secondary sources you cited did NOT use ALL AVAILABLE primary sources to draw their conclusions but used SOME and made the ASSUMPTION that the NNMC continued in use as a name until Isandhlwana and after. I am not responsible for others' research but I do take responsibility for my own. I am happy to be proved wrong (I like to have things right) but there is NO mention, not one word, official or otherwise, of the use of NNMC after 20.12.78 and the first use (officially) of NNH is 6.1.79. There is only one logical conclusion. So, as you write, you may well just have to be disappointed.