It isn't clear that Clemens shared any of "The Mysterious Stranger" manuscripts
with Livy. Neither is it clear whether he intended tp publish any version of
the story. His motives for writing the pieces remain obscure, though I suspect
that his relationship with Henry Rogers lurked behind his Faustian imaginings.
The best book on the manuscripts and, by implication, Twain's motivation in
their composition, is by Sholom Kahn. I believe the title is _The Mysterious
Stranger Manuscripts_; don't recall the publisher. Kahn demonstrates the flaws
in the 1916 Paine-Duneka edition and maintains that Twain favored the version
called "No. 44." It was the last one he worked on, but if he never wanted to
publish any version, the sequence of their composition doesn't matter as much
as Kahn argues. As to the story's 'darkness': there isn't much to laugh about
when you mix determinism with the supernatural, except the grandiosity of human
self-perception, and Twain is pretty funny, although darkly, on the subject.