My thanks to Barb for her very interesting and informative note.
Incidentally, I see now that in my search of Mark Twain texts for "miracle
worker," I failed to consider that phrase as a hyphenated compound. A fresh
search of the texts finds the instances that Barb cites, plus some other
instances, such as chapter 11 of "The Chronicle of Young Satan." None of
these instances has anything to do with Anne Sullivan or Helen Keller, but
they at least establish that Mark Twain used the phrase.

As a sometime collector of Mark Twain quotes, I know that a chief problem of
dealing with unattested quotations is that the mere absence of proof that he
said or wrote something does not constitute proof that he didn't say it. We
would all do well, of course, to refrain from repeating quotes and
anecdotes, however clever, that cannot be substantiated. The more
misinformation is repeated, the harder it becomes to prove untrue. However,
the question of whether Mark Twain really called Sullivan a "miracle worker"
is certainly worth investigating.