I don't believe it's an exact replication of Twain either.
But any license taken seems to serve the interest of being
both interesting and entertaining.
Hal Holbrook did his homework when he consulted Caroline Harnsberger
and through her, I assume, arranged a meeting with Clara.
Also, if my memory serves, he had the opportunity to speak with Bim Pond,
son, or grandson, I can't recall which, of James B. Pond, Twain's lecture
tour booker/manager. Some of Bim Pond's memories served to shape and
perhaps confirm the relative accuracy of Holbrook's pitching, pace,
and drawl from a source with first-hand information.
What a find it would be to discover a wax cylinder somewhere that somehow
survived and could be confirmed as Twain's voice. The value of that would
conjure up Twain's musing on how much a soap bubble would cost if there
only one in the world. (Those cylinders are apparently just as delicate.)
But, as you rightly point out, history plays strange tricks. Who would
the other half of the HF manuscript would surface after so many years?