I also am not familiar with a reference to Guy Kay in HoME, but that does not mean there isn't one.
As the interview explained, Kay was young when he worked on the Silmarillion - he wasn't a novelist yet, and therefore wasn't brought on board for that reason (in particular, anyway).
CJRT has been fairly decent about discussing mistakes in the published Sil that he noticed while working on HoME. For instance, the confusion of whether the Halls of Mandos were North or West, and the ancestry of Gil-galad (the idea that he was Fingon's son was ephemeral, but that was not obvious when CJRT chose that variation - also, it
was the simplest).
Also, he has pointed out where he made purely 'editorial' changes, such as retaining a passage from 'The Fall of Gondolin', but dropping the reference to the battle of 'Rog, outside the walls' - since Rog was clearly an elvish name that would have been altered, later, had Tolkien ever rewritten that story fully. I think dropping the reference to Amarië at Finrod's death was much the same (I don't know that it was justified by any text).
Much of his commentary points out where significant details are introduced, and how the version of the story being presented measures up to the published Sil. I mean, he gives us what we need to see
how he did his work, though he does not always justify the choices he made.
I think it was
Eluchíl who pointed out in a thread on TORc an example of some of the blending CJRT had to do:
- What were the names of Elwing's brothers?
What was their fate?
Who was responsible?
Edit: Found the thread!
What happened to Elwing's brothers?
The stellar post, though, belonged to
Scirocco
There were at least three different versions of these details, and none was clearly 'final' or 'best'. But some decision had to be made, if you were going to tell their story.
Also, the Tale of Eärendil and the War of Wrath was pretty much in disarray, wasn't it? I know that it was sketched at various times, but was there anything approaching a final version of that? I thought (perhaps erroneously) that CJRT had to basically write that himself, though of course basing it on JRRT's writings. The transitions, of course, have been smoothed over by him as an editor, as well.