Subject: As I understand it...
Author:
Posted on: 2015-10-17 10:37:00 UTC
... the difficulty is that while the Professor wrote one of the twins dying at Losgar, he never carried that change through. So the rest of the story still refers to both of them being alive. Relatedly, the narratives concerning the death - specifically two scenes with Nerdanel, one with her prophesying the death, and one with her begging Feanor to leave the twins in Valinor - were never slotted into the story; there's no hint in the Silm narrative as to where they fit. (The latter, for instance, would need to fall between the Oath and the Kinslaying - but the narrative runs at high speed in that section, and wouldn't easily take an injected Family Moment.)
So Christopher excised it, because there was no way to fit it into the story without massively rewriting sections of the Darkening of Valinor. It's a defensible decision, though I do wish he'd come out with an Extended Edition incorporating all that sort of material. (It's been noted that the women of the Noldor get massively cut down in the published Silm.)
Gil-Galad's parentage, on the other hand, has been admitted as a simple mistake: Christopher didn't realise at the time that it was only a passing notion.
hS