I'll give as many sources to what you said here as I can, just so people listen more. Wasn't able to track everything down, so some things are more up in the air, but overall, you're on the money.

[Blog of Dave Strider- February 17, 2010.]

I heavily agree and identify with your frustration. Just because it's "not in the comic" doesn't mean Hussie didn't write it and that it's not worth evaluating. This isn't a case where you can disregard it for noncanonicity reasons, like Post Canon. This was written concurrently with canon, and therefore it reflects the political mindset of Hussie during the time of writing Homestuck. That much is valuable information. What I will point out is that the next paragraph here contains yet another phrase that is rather telling of Hussie's... Racial vocabulary.

Definitely not the most comfortable thing in the world, to put it nicely. Haven't personally seen it get mentioned, and I feel like it's apt to bring up here, since this is the closest I've seen Hussie get to the big Hard R. Typically, they default to Soft A, which doesn't make it less racist, but does have a different... Vibe, so to speak. It's more hateful sounding.

Moving on.

[Andrew Hussie's Formspring- Feb 24, 2010 - Mar 7, 2010. Exact date unknown.]

The fact that Dave is a near exact emulation of Hussie themself in speech mannerisms, down to the lifting of entire chatlogs, is telling. As we've seen from Hussie's past work, he was clearly... excessively comfortable with throwing around the N-word, as a white man. Why would Dave be any different? It's hard to argue that Dave's racism is "not indicative of Dave's flaws, but indicative of Hussie's", when Dave as a character is both a product of Hussie's mind and a direct extension of her. You can't truly divorce the two. Dave is racist because Hussie is racist. Dave is an extension of Hussie.

[Homestuck, Act 2, page 294- June 23, 2009.]

I believe this is what you were talking about in reference to that particular version of the N-word being used, as well as the references to slavery. This is in Act 2. I couldn't find much in Dave's other raps, outside of his fixation on the idea of Black presidents as a novel concept... Other than this.

[Homestuck, Act 5 Act 2, page 2825- November 1, 2010.]

Apparently using the term "Urban" to refer to Black people, culture, art, and music is a recurring Hussie-ism, even in Homestuck. Rose also uses this a couple times, notably more than Dave does. It sees more usage in Problem Sleuth as well- unsurprising, considering it is an older work than Homestuck is.

[Homestuck, Act 4, page 1590- March 17, 2010.]

[Homestuck, [o], page 3879- July 2, 2011.]

[Problem Sleuth, page 62- March 15, 2008.]

TL;DR: I don't think there's any real excuse to leave Hussie's racism out of genuine discussion of the contents of his works. It's obvious. The only valid defense for leaving it out of analytical conversations is ignorance, because... Well, some people just aren't that observant. Homestuck does not exist in a vacuum. Hussie did, in fact, write this, and Hussie is, in fact, bigoted. Bigotry shows throughout their works not because they're spinning a grand narrative of overcoming your moral failings, but because they kind of just suck. Dave isn't an exception, and shouldn't be divorced from the context of having been written by Andrew Hussie. There's no debate to be had over the existence of Hussie's bigotry, because contesting such a clear cut fact is pure stupidity. It's like arguing that evolution isn't real because you, personally, do not understand it, or don't find it comforting. Hussie's bigotry is one of the only things we can be 100% sure is a real part of his personhood, rather than another figment of his ironic online persona. The level of "Well, Actually"-ism people will engage in over this is mind-numbing, quite frankly.