Insults don't help you make your case.
Regardless, you must have grown up in a different part of the country than I did. My home town had a population of about 6,000. Of those, only a dozen or so were not white.
Even in the 70's, many locals still used the term nigger to routinely refer to black people, as in "a nigger came into the store today." It was a common part of the local dialect. The usage of the word was certainly insensitive, but it wasn't meant to be malicious. Perhaps you find that hard to believe, but it's true. That's just what life was like in that part of the country not long ago.
Because of that, I don't believe that Twain's repeated use of the term nigger was intended to be harmful. That's simply the term that Huck would have naturally used given his upbringing. Anything else would have made the dialog sound artificial. If you go back and read the book, Huck is still calling Jim a nigger even in the last chapter. Despite being friends and having respect for Jim, he doesn't change to using some other term. How, then, do you back up the conclusion that Twain intended for the word to be viewed as intentionally injurious?
The book was absolutely meant to highlight the belief that everyone, even non-whites, should be free. But Twain wasn't trying to change anyone's vocabulary. And I don't believe he would care overly much about the use or avoidance of any specific words in his book other than free and freedom.
And before you go saying my hometown had nothing in common with the world Twain lived in, it was only 20 miles from where Twain was living in Hartford when he wrote the book. The people living there are exactly like the audience he was trying to reach when he wrote the story.