Fornekja Vs. Forneskja

In follow-up to my previous post, Edred Thorsson spells the type of witchcraft I've called fornekja - fornekja. I couldn't find fornekja on the regular internet, so I went to Google Scholar and found it (here) spelled forneskja:

The term ‘witch’ itself has a great range of meanings in Old Icelandic, and it is not just one word, but a series of concepts which may be translated as ‘witchcraft’. These are fjölkynngi, fyrnska, and forneskja. The latter two have the connotation of ‘old heathen time’ and ‘old lore’. 

Since Edred Thorsson is an expert in old Germanic and English languages, while the writer of the article (Astrid Ogilvie) is educated more in climatology and environmental research, I'm more inclined to view the proper English spelling to be fornekja.

Still looking into the discrepency in spellings for this word.