First off, awesome article. Very informative and great content.
But I think you're ignoring the broader assumptions and implications that makes a person think "Well Blacks sold slaves too!" is a great comeback.
Something I hear a lot from racist Whites to justify their prejudice is "Well X was no angel," where X is virtually any public Black figure or group. The speaker's assumption is that the listener has not actually internalized that Black people are people and are capable of being complex, flawed, or otherwise human. Instead, they project their own selfishness and lack of empathy upon the listener and assume they're only feigning compassion for personal benefit, and that Blacks are, subsequently, just means to an end, and revealing that Ahmed Aurbery had unpaid parking tickets or whatever is some startling, earth-shattering insight that will completely cause all of their beliefs to collapse.
Meanwhile, anyone with a heart or a brain instinctively understands, just as a matter of basic right and wrong, that whatever dirt you dig up--assuming it's even true in the first place--has zero influence on the basic problems with racism. Not morally, not rationally.
As if I'm suddenly gonna become okay with the torture, murder, and subjugation of tens of millions of people because you showed me a Facebook photo of Trayvon Martin jaywalking.