This won't resolve the question of whether it's Kosher to alter "In a Mist," but the debate got me thinking:
There's an old Zen expression: "Don't mistake the finger pointing at the moon for the moon itself."
We may see "In a Mist" as a finished composition, but it's pretty clear that Bix himself didn't. A comparison of his OKeh record to the published music clearly shows that Bix continually altered the piece, until he went on to something else. In Bix's mind, "In a Mist" was malleable and subject to constant change.
Since early in my appreciation of his music, I've wondered about Bix's compositional/improvisational process, which gave us, as incidental byproducts, "In a Mist," "Candlelights," "Flashes" and "In the Dark," not to mention his recorded cornet work. What happened between takes captured my interest as much the records themselves.
As I noticed how totally Bix was immersed in what he was doing, that became my aim, too. Bix has been a great teacher. He taught me not to copy him, to play his music note for note, but to evolve a fluid process, just as he did, that opens the door to magic. Thus when I practice music, my focus is more on that process, the HOW of the playing, than the WHAT I end up with. Of course, since it's my process, it sounds like me and not Bix, but it's the same impulse, persistently applied.
So, yes, "In a Mist" can stand as a fait accompli, set down for all time; or it also may be seen as a snapshot (or two) of something in process, that never sat still until its creator departed the scene.
For my two cents, 'tis far better to gaze into the indeterminate distance, possibly at the moon, where Bix is pointing, than at his mortal finger.