Posted mainly as a future reference for when these arguments come up again (as they always seem to when people discuss this particularly foul chapter in American history).
Seven Lies, Half-Truths, and Irrelevancies People Trot Out About Slavery—Debunked
A certain resistance to discussion about the toll of American slavery isn’t confined to the least savory corners of the Internet. Last year, in an unsigned (and now withdrawn) review of historian Ed Baptist’s book The Half Has Never Been Told, the Economist took issue with Baptist’s “overstated” treatment of…
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As defenses of slavery, each of those claims is a fallacy, which is to say that even if they were unequivocally true, they would not support the claim that slavery was morally acceptable. The first four and the sixth are various forms of the "tu quoque" fallacy (two wrongs don't make a right). I would argue that the fifth and seventh are appeals to popularity, but it depends on exactly how you analyze the arguments.
+David Newman I think they are less defenses of slavery per se (except amongst the fringes), more defenses of slave-holding South.
I'm not sure what the conclusion would be in that case, but those are still fallacies as support for any claim regarding slavery or the south that I can think of.
+David Newman Well, if the argument is that the South wasn't exceptionally bad for being vested in chattel slavery because [etc.], it makes a certain amount of sense … except that then gets rhetorically elided into the conclusion that the South wasn't bad at all and slavery wasn't all that bad either, which, as you note, these arguments don't address.
In short, yes, it's fallacious, but a bit subtly clever, too.
OK, so now I'm thinking in terms of the principle of charity. If the argument is that slavery in the south was not uniquely evil, then first four and the sixth claim are arguably not fallacious (though thenn the question of whether or not they are true arises). If the argument is that slavery in the south was not widely recognized as evil at some time or place, then the fifth and seventh claims might not be fallacies (but again, then their truth becomes an issue). But I think neither of those claims are in dispute, so it's hard to imagine that anyone would present claims in their favor.
I think the really clever thing would be to get people to think the question is whether slavery in the south was more or less evil than other examples. If that is what defenders of the Confederacy are debating, I think they arguably missed the boat.