For me, Tolkien-published* text is canon.
And as the posthumously published stuff seems to raise the very question, again for me, that seems an easy division. Were we ever supposed to see any of these texts
in the forms that we find them today? Even those texts that seem finished don't have that final stamp of author-approved . . .
Quote:
"The desire to revise, improve, and polish was characteristic of Tolkien. His obituary in the Times of London, obviously written by someone who knew him well, says that "his standard of self criticism was high and the mere suggestion of publication usually set him upon a revision, in the course of which so many new ideas occurred to him that where his friends had hoped for the final text of an old work they actually got the first draft of a new one”.
Christina Scull, The Development of Tolkien's legendarium, Tolkien’s Legendarium, Essays on The History of Middle-earth
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* Also, any "Hobbit-holdouts" (there appear to be some who do not hold any edition of
The Hobbit as canon) will have to deal with Tolkien's own statements about even the first edition Hobbit being an internal work -- statements made in the canon itself (here The Lord of the Rings).