The film looks like fun. Yes, MT stories published before 1923, as was
The Stolen White Elephant, are PD.

This is the way the Bancroft folks phrase their paragraph about their
costs and the licensing revenues , which seems fine to me:

 "The scholarly and technological work of the Mark Twain Project
and Mark Twain Project Online is costly. Revenues from  licensing
benefit the Mark Twain Foundation, which in turn makes annual grants to
organizations that promote the study  of Mark Twain and his time,
including the Mark Twain Papers and the Mark Twain publishing program of
the University of  California Press."

http://www.marktwainproject.org/copyright.shtml

Also provided at that link is a contact name and address for the
Foundation:

 To write to the Mark Twain Foundation:
 Richard A. Watson
 Chamberlain, Willi, Ouchterloney, & Watson
 575 Eighth Avenue, 16th Floor
 New York, NY 10018

DDD