So, I've been delayed getting this post up by three smallish projects I wanted to get off my desk. Don't want to bog down and get distracted again so I'll try to keep this short.
First off, thanks for the many interesting and well-informed Comments.
My own take on the the respective roles of Christopher Tolkien and Guy Gavriel Kay in putting together the 1977 SILMARILLION is simple: I don't know of any evidence that Kay wrote any of it. And I wd be surprised if he did.
I think it far more likely that Kay helped in the sorting and sequencing of the manuscripts, that all-important stage of surveying just what materials existed for each chapter or associated work, after which Christopher wd have decided just which Ms he wd use as his text(s). I think Kay also served as a sounding board, whereby Christopher wd occasionally ask his opinion on specific points. Sometimes Christopher took Kay's advice, and we know of one or two specific examples. But the decisions wd have been entirely Christopher's to make.
In short, I see the Christopher/Kay working relationship as paralleling Christopher's working relationship with Taum Santoski a decade later on the LotR Mss, probably because I was there to see the latter.
By contrast, Kilby's role a decade before CT/GGK 's work was quite different: every other day or so for a month Kilby dropped by Sandfield Road and picked up a typescript of a given chapter of SILM, talking about the preceding chapter with Tolkien*, and repeated the process day by day. So what he saw was the latest version of the component pieces (including associated documents like the ATHRABETH), what I call 'the 1966 SILMARILLION'. Of the three --Kilby, Kay, and Taum -- I think Kilby had the least input and Kay probably the most, with Taum between the two, closer to Kay than Kilby.
And I'm grateful to them all.
--current reading: Richard Sala graphic novels (EVIL EYE, THE CHUCKLING WHATSIT)
* though given the interconnectedness of everything in Tolkien, in practice they spent more time talking about the legendarium than that day's specific piece.