[Galin]

If you're asking me WCH...

... to me it wouldn't be canon even had Tolkien himself written it. Which doesn't mean I don't believe Tolkien did not intend to finish and publish his Silmarillion however (interesting line "as it has come down through the Numenoreans" by the way, considering the date).

"Canon" for me is an internal affair, a guide for Middle-earth the subcreation. Tolkien himself wrote that Ambarussa perished in the fires of Losgar for instance (as I know you know). I believe JRRT intended to make this true within his world, and so my personal Silmarillion includes this as a "real" event...

... does yours? Does Christopher Tolkien's imagined personal Silmarillion include the early death of this Elf? Maybe, maybe not. It's not canon, and despite that it seems very much to me that Tolkien desired this new revision, I can't know it with the same confidence that I know Aragorn is the son of a man named Arathorn, or that Felagund became Finrod Felagund in the second edition... the line is still there, but if Tolkien himself had published the early death of Ambarussa, I'm thinking the readership at large (includes folks not born yet) would hold this as true, it wouldn't only be true according to my personal Silmarillion (or possibly other people's too), due to the weights and measures that I (or some) accord to writings outside canon.

Maybe it's my choice of nomenclature. I use "unpublished" texts (including parts of letters) to expand my personal Middle-earth, I just consider them with the same line drawn in the sand that I believe Tolkien drew, or that Tolkien should draw due to the art of subcreation: already published work comes first -- it's really the only "canon" JRRT needs to mind I think, both when he is trying to be consistent, and when he purposely introduces an inconsistency (or even gives into a niggle).

It's quite interesting when Tolkien plays the game you raised, treating still private texts as if they are already in print, and "known", but for myself I doubt that if a given scenario proved unworkable for some reason, Tolkien would not bend to putting story above a concern that he knows he has constructed as a "must consideration" of sorts.

There's an art to being inconsistent too (not that you said otherwise), and this type of inconsistency is not the mere act of revision when working toward a final version of something.