I have no scientific proof for this, but I strongly believe that Twain's
use of the N word was a first cousin of the following piece from The
Mysterious Stranger:

"But it was Father Peter, the other priest, that we all loved best and
were sorriest for. Some people charged him with talking around in
conversation that God was all goodness and would find a way to save all
his poor human children. It was a horrible thing to say, but there was
never any absolute proof that Father Peter said it."

In this case, Twain knew that his audience had advanced well past the
middle ages, and Father Peter's remarks seemed pretty reasonable, even
though the narrator is completely unaware. So too, in HF does he throw
the N word around liberally with a wink to his audience of Americans in
the 1880's who had advanced far enough since the Civil War to know that
slavery was an evil and that it was wrong to downgrade a people because
of their race, even though Huck the narrator was confused about these
matters.

Terry Ballard
Quinnipiac University