I think this is an excellent and very important article.
I agree that the films use of the n-word is beyond excessive, but I wouldn't necessarily consider the film hiding behind the context of slavery so Tarantino can write the word into his script. He's excessively used that word in a number of his other films, and while his portrayal of black culture does border between fervent admiration and exploitation, I don't think thats the case here.
I think this film's point is mainly to emphasize that slavery was despicable and morally bankrupt, and harness that moral bankruptcy in order to create a perfect revenge fantasy a la Inglorious Basterds. It is, as most Tarantino movies are, made to be fun and entertaining. In this case, its so successful because it can guide white people and black people alike into rooting for a slave's violent revenge. In Tarantino's films, we don't need to be above gratuitous violence as our tool to back our moral cause (in this case abolitionism). We can agree that people as despicable as Jackson and Dicaprio's characters deserve such bloody, violent demise, and we can have fun watching such demise.
In the film, the use of the n-word is rarely used for comedy. The exceptions would be Jackson's and Dicaprio's characters, who are the most chillingly evil in the film. The comedic use serves to exemplify their cruelty, and what's funny about it is the extent to which Tarantino portrays their evil. The underlying point is that by using this word so often, so commonly, and as humor, the film portrays them as villains.
And the Foxx-Jackson scene is a poor example. Jackson's false, contempt-worthy use of the word shows how contempt-worthy the character is. Foxx's role in this movie is tough. I didn't interpret his character as one obsessed with being gangster, but rather as one fixated on revenge well beyond the point of smoldering furiosity. His repeat of the n-word to Jackson is an embodiment of that furiosity, and him making fun of Jackson's character, taking pleasure in killing him. Foxx is toying with the character.
I think at the end of the day this film makes society less racist by revealing, in a fun, entertaining way that will certainly strengthen the convictions of many lay viewers, the inescapable injustice of slavery.