I love this question of what is authoritative; I would welcome any links people have to the debate. I was amazed when I learned how much editing was required to form the coherent Silmarillion we now have; not that CJRT made up anything, but there were so many drafts and so many differing fragments. I was so glad CJRT decided to publish the HoME series, and you can see if you read that how much he had to pick and choose and stretch and tug and cut in order to achieve a coherent Sil. But really wouldn't trade the Sil for anything... I do love it.

Do scholars here have some reading suggestions for us, either academic articles or links, on exactly what we should know about the canonicity of the Sil? Thanks.

"It is perhaps not possible in a long tale to please everybody at all points, or to displease everybody at the same points; for I find from the letters I have received that the passages or chapters that are to some a blemish are all by others specially approved."
--J.R.R. Tolkien
  Forward to the 1965 Ballantine edition of "The Lord of the Rings"