Between the Far and Near Krekalands Jon found some islands that he liked. Upon the largest he went into the wood between the mountains (to) build a burgh.
The largest of the Ionian Islands is named Cephalonia. No one really knows from what the name is derived, but one suggests that it is from the legendary hero Cephalos, who reigned in Phocis in Central Greece and came to the island as a refugee from Athens. Others claim that because 'Cephalos' is derived from the Greek word for 'head', the name means 'an island with a head' - referring to its form.
At one time, when I studied the geography of the Ionian Islands from my world atlas - where all placenames are written in the individual countiries' own languages, I found that the Greek name of the island is Kefallinía (Κεφαλληνία).
Wasn't it a natural gesture of Jon to name the largest of the islands in his new kingdom after the burgh-femme whom he had saved and recently brought to the Mediterranean? It was also just the seamen who had given Minerva that name (62/10-11). As the basis of the designation gradually sank into oblivion, neha- became keha-, because it was easier to pronounce. The first syllable, neha- - an h between two vowels - isn't easily pronounceable for anybody. In people's everyday speech the word was simply deemed to be changed into either 'Nefallennia' or 'Kefallennia'; and in the name Cephalos we find almost an attestation that the Greeks would naturally choose the k instead of the n. Moreover, the Greek η (êta) is pronounced 'ê', like in the English 'hey' - which gives us 'Kefallênía'. In the last instance it is simply the same word as Nehalennia.
Also Raubenheimer makes the hint that there might be "a relation between Cephallenia and Nyhellenia" (p. 140). It doesn't exist one single name another place on the globe which resembles that word more than this one - probably not even other placenames that could have had this name as their base linguistically. I think this also suggests that Nyhellênja was pronounced more like Nehalennia by the ancient Frisians.
Indeed, when the Greeks are writing, they utilize more versions of the name, and the most regularly used one is Kefalonia (Κεφαλονιά) and Kefallonia (Κεφαλλονιά). English speakers usually write Cephalonia - but also Kefalonia, Kefallonia, Cephallonia and Kefallinia.
Homer was the first who used the term Cephalites, and he then alluded to Odysseus' people on several Ionian Islands. This indicates that Jon's people may have regarded Nyhellênja as their spiritual leader.
You go from N to K and from H to F to change Nehalennia into Kefallinia. I think that's a bit farfetched.
And no placenames similar to Nehalennia? I remember I posted names (but not placenames) of several Germanic/Celtic goddesses with almost the same name.