My favorite description is that a Southerner will never insult you accidentally. We're acutely aware of manners, so if a Yankee insult is a bludgeon, a Southern insult is a stiletto. By the time you figure out you've been insulted, we're already on the way home.
As an example, my grandmother recently told me a story about her grandmother, one of the Athens Howards, who was quite acid-tongued by Southern standards.
She had a party, or afternoon tea or some such. One local young lady, who was quite homely and not too bright but fancied herself a beauty, coquettishly piped up, "Who'll walk me home? It's geting so a pretty girl isn't safe walking home alone these days." I can only imagine that she was batting her lashes and flipping her hair as she said it. Home was about a couple hundred yards down the road. There were no immediate takers.
My great-great grandmother said, "You don't need to worry about that, dear. The Lord has placed his protective arm around you, and I'm sure you're in no danger." I don't think the young lady ever figured out she'd been, in the modern vernacular, dissed in front of everyone.
I've learned something by observing my mother-in-law - a nice lady but one who had to survive by scraping and fighting. She can tellyou to go to hell in a figurative sense but will say it by laughing and smiling, and you don't even realized that you've been insulted.
You are correct that polite Southerners are subtle in their approach of offending when they mean to offend.
I remember one man describing another man as "a shabby little man" which I thought was a wonderfully sly comment!
... I don't think the young lady ever figured out she'd been, in the modern vernacular, dissed in front of everyone.
That goes a long way to explaining why Northerners don't trust Southern courtliness. If it's hard to tell when you're being mocked or insulted, it pays to be wary.
Scott Fitzgerald wrote a story in which a Southern belle complains that the Middlewesterners she lived among were "canine" -- straightforward, unironic, dogged. Southerners, by contrast, were "feline": languid, nonchalant, ironic, and catty. So it's not surprising that people so different would fight like cats and dogs.
But the girl your ancestor dissed was also a recognizably Southern type, the flirtacious belle who's oblivious to the impression she really makes, so it's hard to generalize about these things. You can find acid-tongued and thick-headed people everywhere.
As a more general observation, I can't help noticing how often these threads start out: "They are such snobs. They think we're all stupid, bigoted, and crude," or words to that effect, and end by saying in essence, "The really crude, stupid, and bigoted people are in South Boston or Bensonhurst, New Haven or Bridgeport, Cleveland or Detroit."
People don't seem to get that they're doing just what they attack others for supposedly doing to them. It takes a while to realize that it's not about truth or justice or manners. People just want a chance to vent, whether to complain about unfair treatment or to treat others in the same manner.
Another thing is just how much Northerners have picked up on Southern cues in these matters. New Yorkers and Bostonians didn't invent terms like "redneck" or "White trash." Such expressions were used by some Southerners against other Southerners.
The association of some accents with negative characteristics is something you can see in both North and South. A pronounced "lower-class" Boston, New York or Philadelphia accent is a disadvantage in the upper circles of those cities, just as a back-country Southern accent will get you shown to the tradesmen's entrance in Atlanta or Montgomery or New Orleans.
Some Northerners (and some Southerners) may generalize from specific Southern country accents to all Southern accents, but they didn't invent those associations or characteristics. You have to look further South for that.
It's not that well-to-do Northerners didn't have insults for their poorer neighbors back home. They did. Or that they needed to learn condescension from Southerners. They didn't. It's just that class is an important part of what people are talking about, and it's often ignored. If someone has an accent that would get them thrown out of an Atlanta or Augusta country club, they shouldn't complain that Northern prejudice is the main problem.
Maybe the point is that it's not "they the oppressors versus I the oppressed." Each of us plays both roles. The Northerner who puts down Southerners, probably puts down other people with accents when he can get away with it, and is in turn looked down upon by others. The Southerner who complains about Northern animosity may face something similar from other Southerners, and dish it out to Northerners and other outsiders when he can get away with it. Acknowledging that might help these threads to make more sense and to more accurately reflect whatever it is that's going on.