Nom, the
Numenoreans are credited with making 'mythological' mistakes' i.e. the sun and the moon and they wouldn't have made factual mistakes. The Sun+Moon creation etc are embroiled in
Mannish mythology and therefore they would have introduced their own ideas and mythos into this.
Gil-Galad was a prominent figure in the S.A and embroiled in
Numenorean affairs therefore his parentage would've been well known, especially amongst the learned, who undoubtley compiled
the Silmarillion. Or taking the account that
Bilbo was also a 'compiler' of the Silmarillion-he would've obtained help from
Elrond and other Elven lords and they would've known about his parentage.
In his final drafts,
Tolkien had not intended for
Fingon to be Gil-Galad's father no more so then for Gothmog to be son of
Morgoth.

C.T didn't introduce Argon etc because they were characters that came from essays. C.T wanted to make a Silmarillion based on the Annals of
Aman and
Beleriand and the Early/Latter Quenta's to avoid confusion, though he at times adopts saying from certain essays into the
Published Silmarillion
Why did Christopher Tolkien choose to use the versions of
Ainulindalë and the
Quenta Silmarillion that he did in the Published Silmarillion?
Maedhros, you said that the Quentas in HoMe 10 and 12 are more complete
There is no Quenta in HoME 12, that is appendices drafting and essay, I think you mean HoME 11-yes? I think the most 'complete' Quenta's are 4 and 5, which are pretty large in contrast. The Quenta in HoME 10 deals with the Eldar in Aman (i.e. a time before they came to M-E) and this is reflected by the
Annals of Aman. (I'm unsure as to who the heck wrote the Annals of Aman since in
Tale of Years Tolkien states Quennar Onotimo as the compiler of the records of time but at the start I'm sure it says Rumil compiled them, so maybe Onotimo compiled the record of time in Beleriand.)
The Quenta found in HoME 11, which deals with Beleriand is not very big and it is rather incomplete, and C.T had to take a lot from the
Grey Annals as opposed to the prose version of the Quenta, which makes for mistakes and lack of description that could be found in the prose version.
from what texts do we get the Published Silmarillion
Early Quenta Silmarillion-HoME 5 (Mainly from the Tale of
Earendil,
War of Wrath etc since Tolkien didn't get round to doing this again)
Annals of Aman, Latter Quenta Silmarillion 1 (HoME 10)
Grey Annals, Latter Quenta Silmarillion 2 (HoME 11)
Possibly other essays...
(e.g. a library or some sort of archive - just like the original LOTR manuscripts were sold to Marquette University in Milwaukee)
Those were early drafts, do you really think C.T would sell/lend final drafts? I dont remember him claiming any Silmarillion related works were sold/lent.