Twain actually wrote multiple versions, each unfinished, but all
included Satan as a character. One version, The Chronicle
of Young Satan, is about the nephew of Satan who lives in an
Austrian village in the Middle Ages.
Another version, Schoolhouse Hill, has Huckleberry Finn
and Tom Sawyer involved with Satan, who is referred to as "No. 44".
It takes place in America.
The third version is called No. 44, The Mysterious
Stranger, taking place in the Middle Ages as the first version
did. This version introduces the duality of a person, the
"Waking Self" and the "Dream Self". It has an ending whereas
the other two are unfinished.
Paine had possession of Twain's unfinished works after his
death and added an ending he thought Twain had intended. After his
death, the manuscripts were released to the public and critics
found Paine had chosen the ending from a different version of the
story and changed some of Twain's wording. The edition to
which you are referring is called the scholarly edition, considered
to be what Twain would have published if he had lived.