Would MT/SLC have approved?  Good question.  Consider these
(perhaps pertinent) comments from a letter to William Clemens on June 6,
1900, regarding a biography William Clemens wanted to put together on SL
Clemens while SL Clemens was still alive:
        "A man's history is his own property until the grave extinguishes
his ownership in it...These laws...allow no unauthorized remarks to be put
into his mouth."

Then, too, there is this later letter to Will Clemens on August 27, 1900,
which is not directly related to rewriting a life, but which does touch
on the issue of ethics ("etiquette"):
        "I am afraid you did not quite clearly understand me.  The
time-honored etiquette--new to you by means of inexperience--of the
situation is this: an author's MS is not open to any uninvited
emendations.  It must be accepted as it stands, or it must be declined;
there is no middle recourse.  Any alteration of it--even to a word--closes
the incident, and that author and that editor can have no further literary
dealings with each other."

In retrospect, perhaps Clemens should have written the above letter to
Albert Bigelow Paine, eh?