Christopher Tolkien says in THE PEOPLES OF MIDDLE-EARTH that the notion of Gil-galad being Fingon's son was never more than "an ephemeral idea". It is simply impossible for him to be Fingon's son. He was not always Orodreth's son, but he was almost always a member of the House of Finarfin, originating as Finrod Felagund's son.

On the continuing question of kingship and inheritance (which may never be satisfactorily settled), I found the following entry in "The Etymologies". It is therefore dated information but seems to me to be worth mentioning, as probably having been retained by Tolkien into the 1950s and 1960s. This is a subentry under "TA-, TA3-":

  *taro  king: only used of the legitimate kings of the
  whole tribes, as Ingwe of the Lindar, Finwe of the Noldor
  (and later Fingolfin and Fingon of all exiled Gnomes).  The
  word used of a lord or king of a specified region was
  aran (âr), Q haran [see 3AR].  Thus
  Fingolfin taur egledhur 'King of the Exiles' [see LED],
  but Fingolfin aran Chithlum 'King of Hithlum'.  Q
  tár (pl. tári).  N taur,
  Ilk. tôr, only used of Thingol: Tor Thingol
  = King Thingol.

If I understand the language relationships correctly, "Q" represents "Qenya", the predecessor language for Quenya, spoken only in Aman. "N" represents Noldorin, the predecessor language for Sindarin, spoken by the Exiles, and "Ilk" represents Ilkorin, spoken by the Elves of Beleriand (and perhaps other lands, but mostly Thingol's people and the Danas, who became the Green-elves of Ossiriand). Ilkorin would have become the Doriathian dialect of Sindarin.

As I've pointed out elsewhere, there is not a 1-to-1 correlation between these languages and the later languages, but they are very similar and Tolkien did in fact retain the name "Noldorin" for what became "Sindarin" even up until he was working on the Appendices for THE LORD OF THE RINGS, and only then made the name changes.

An entry "NDEW-" for "follow, come behind" does exist in "The Etymologies", and one of its derivations was "Dior, successor (i.e., of Thingol)", but I don't know if these entries are contemporary, or if the omission of the title "Tor Dior" was due to oversight or in some way deliberate (and if so, was it deliberate only in the sense that Tolkien felt one example was sufficient, or in the sense that Dior did not succeed Thingol as Tor but only as Aran?).

This entry, however, does indicate that Dior's name was more a literary convention than anything else, although it would not be impertinent to infer some sort of prophetic association may have been intended.

An entry for "khil-" is carried over into the Elvish dictionary in THE SILMARILLION, and it is said to mean "follow". Although "Eluchil" is translated in its own index entry as "Heir of Elu (Thingol)" a more literal translation would be "successor of Elu". Dior's early name was "Dior Aranel" (Dior Royal-elf) and he is only said to have later been called "Dior Eluchil". Use of "heir" interchangeably with "successor" here seems to indicate an understanding that he who followed in the other's footsteps was taking up or receiving the heritage left of the predecessor. On the other hand, "tarkil" comes from "tara-khil", for which no meaning is given, but which I would guess should be translated "a follower of the High King" or possibly "Heir of the High King" (it depends, really, on who Tarkil refers to -- it was the personal name of a Gondorian king, but it was also the root-word for the Orcish name "Tarks" for the Men of Gondor).

In neither case, however, is the name appropriate for a king of Gondor, except in a figurative sense. The High Kings dwelt at Annuminas in Arnor.

Returning to Dior, I cannot determine from the evidence given in THE WAR OF THE JEWELS when "Dior Eluchil" is supposed to have first been used among the Elves. One entry in the much revised "Tale of Years" seems to imply that it was only given after he reestablished "the kingdom of Thingol". Maybe in this phrase we can all look for some sort of compromise.

It is apparent, however, that Dior had to have travelled to Doriath well before he restored the kingdom. Tolkien abandoned the idea that his wife was Lindis of the Green-elves and he made her Nimloth of Doriath (of Thingol's kin, no less). And the "Tale of Years" keeps saying that Dior RETURNS to Doriath to restore the kingdom. He must have visited Doriath around the year 497 or earlier (this is the date given for his marriage to Lindis of Ossiriand), or during his mid-twenties. So Dior would have been in Doriath around the time Turin was in Brethil, but they apparently never met.
------------------
Xenite.Org: Science Fiction and Fantasy