"find it fascinating that your entire race has essentially been written out of American history - simply because it complicates the narrative, it seems." 
 
Thanks, but more on "my race" a little later. The media conception of race nowadays is embarrassingly type-cast. For instance, a devout Catholic nun, Herietta DeLisle http://creoleneworleans.typepad.com/creole_folks/2006/10/betryal_of_moth.html,  
was famous in her day, 1810s-20s, for her work among the poor blacks and pocs in New Orleans. In appearance she was completely European, quite pretty and well-known as being of mixed blood. If current Hollywood got hold of her story she'd probably be played by Whoopie Goldberg. Whoopie's fine, but Henrietta DeLisle she ain't. Modern race politics conflate black with mulatto or even quinteroon (look that word up for an education in math and race) and is quite a problem even today among people who still identify with the "people of color" label. I don't. It was my dad who whose parents were from that group and they left the area long, long ago and are long since deceased. Dad didn't know and didn't care though some of his brothers and my cousins did eventually. My mother is Euro-ethnic. I don't look black by any stretch of the imagination and while I've met many blacks I like a lot, I don't identify with them. Even many poc's who do look a bit "colored" are not well received by blacks. There's no point of claiming that identity and if I did I'd just be looked upon as ridiculous. A DNA test done showed my genetic heritage as 99% Caucasian and 1% American Indian. Who that Indian was I have no idea though I know names and dates galore for the blacks, pocs and whites in my genealogy. Yet in spite of all those blacks, I have no African genes in my pool that manifest. I had figured it would be about 6% at least.  
Still, it is a fascinating history. The interplay of black and white and Indian was far more complex than most people understand. 
One interesting thing I found out. The French priests in the early days used to encourage Frenchmen to marry Indian women, white women being in short supply and often demanding French born husbands. The priests opined that there was little difference in color between the Indians and French and that such marriage was better than illicit fornication. Many tribes of eastern seaboard Indians were different in type from plains Indians and their origins have been a source of speculation. Some think they may have middle eastern roots. Anyway, "Color" as shorthand for race was an important, though not always decisive consideration, and the Indians seemed more integratable with whites than did the blacks. I definitely got that impression in my studies of the histories. The black women who ended up as wives of white slaveholders -- interracial marriage was legal under Spanish law, extant during the 1770s-90s or so -- must have been exceptional people. Some of these interracial unions l
More....

Concerning slavery: the fact that slaves were associated with certain work probably did a lot to discourage progress. If slaves had been profitable then the industry of the South would have rivaled that of the North. It did not. They did work that was an extention of the farming, shipping and various industries associated with agrarian societies. 
 
Some of my mulatto ancestors owned an Island in the gulf of Mobile that had been claimed by their French forebear in 1710. This family was famous for their orange groves and for hosting Andrew Jackson during the war of 1812. 
Mulattos in Mobile (AL) had a monopoly on ship carpentry by the mid-1800s. They did jobs related to shipping, fishing the waters, and growing things on land. Most of my mulatto ancestors who were not land owners (inherited from the white fathers) worked in ship trades and in sheet metal factories. Well into the 20th century most did not pursue higher education. If they wanted that, they left for the north and west. The ones i know of looked entirely white anyway. They probably thought the whole identity was sort of silly as the family line descended further from the original mixed couples. About the work they did: There was a stigma attached to such work in that area. There is a true story of a white family whose son had befriended a school mate. When they found out the boy's father was a bricklayer the white family ended the association because--although the boy looked white--bricklayers with French or spanish names were all mulatto (and a few blacks) in Mobile.  
This is sort of crazy because plenty of European immigrants, Irish, Italian, etc., were working blue collar trades in Mobile and new Orleans. But if someone had a French or Spanish surname and was in such work, they were assumed to be of mulatto heritage. 
The mulattos had their own fire brigade in the 1850s, and it was only open to mulattos. They had a spectacular mechanical light display for Mardi Gras, 1855, which they had had sent from Philadelphia. 
So yeah. I think we know "people of color" (actually it was the mulatto class rather than blacks who were out and about the most) did more than pick cotton and make cornbread. However, the agrarian societies of the South were not a million miles from the way of life in some ancient Roman enterprises or a feudal serfdom more than even the most humble Massachusetts mill town.

"most of spanish south america (exclude cuba) outlawed it in the early 19th century after independence. slavery was one of the reasons that the texans revolted against mexico. OTOH, since you are probably thinking of brazil, which had most of the slaves in south america, most south american slaves were freed in the 1880s." 
 
Yeah, razib. I thought there were a few more countries than just Brazil that still had slaves. Guess I had the wrong continent. It's Africa that still has slavery as any number of Sudanese and Mauritanians can tell you.

"Slavery was most profitable in a cotton plantation, but depending on the price of a enslaved human, it was not impracticable in other areas (e.g. mining)." 
 
history is full of what ifs, but the fact of North American slavery is that it was almost all confined to privately owned, family farms.  
If they were really big enterprises, they were called plantations. Mulattos and even a few blacks owned slaves. I know--my ancestors were among them. I have seen records of my "griffe" grandmother, 5x removed, having her slaves baptized. She herself had been a slave, became a common wife of her "master" who freed her and her many children. She inherited the slaves and she lived to be about 90, dying in the 1840s. Some blacks make excuses for these cases, saying the blacks and mulattos who inherited slaves did so to free them. Maybe some, but only the Civil War freed most slaves, even when the owners were black or "of color." 
Fairly common story among mixed people in the Gulf area. When you think about, many of the slaves came from slave holding societies in Africa. Some had Muslim backgrounds (the Arabs were into slave trading big time.) If it was god's will you were a slave, so be it, they may have thought. 
Slaves made wealth for individual families, though large plantations, esp. in the Caribbean, did make sugar very cheap.  
Most of the North was opposed to slavery as was Europe. The pressure of Great Britain on the United States was as strong as that put by the U.S. or South Africa in 80s, or Iraq in the 90s. 
Slavery almost certainly--though one can never know such things for sure--would never have spread beyond the areas it already existed. The terrible bloodiness in Kansas during the 1850s, involving mostly whites either opposing or supporting it, undoubtedly had a discouraging effect. 
In the long run, the change of economy would have ended it. Most historians have said it would have ended by the 1880s, which is about the time most South American countries ended it. If you're the slave, though, that's a long time to wait.

"South including the lowland and upland southerners plus (in many respects) the interior west south of Montana. After the Civil War a lot of Southerners headed West." 
 
The migration tended to be lateral. The emmigrants didn't usually veer strongly to the north or south, but went straight west. Thus a lot of New Englanders and upstate New Yorkers ended up in the Dakotas and, of course, Oregon and Washington State, which explains a lot. 
The southwest got the real southerners, as Lonesome Dove so well portrayed the defeated Confederates and womenfolk taking off for Texas, Arizona etc. The mid-Atlantic dwellers went to the central western areas.

At first when I read your post, I thought -- what!Fitzgerald's Babylon! The Great Satan! A Quaker! 
But I think you're on to something about this country becoming "Quaker", or at least Quakerish in ideation. 
A little genealogical research on a family line I'd thought was all southern creole (French,Spanish, Irish, British and African)turned up surprises. It had offshoots that led to --- Quakers! I already knew about a bunch of New Haven, Connecticut families, who make me a distant cousin of Aaron Burr, Jonathan Edwards and Harriet Beecher Stowe among other illuminaries; but thousands of Americans are cousins in various degrees of these folks as colonial Yankees were extremely prolific and had comparatively low child mortality. However, the Quakers were a surprise. What the hell were they doing in Mobile, Alabama? Apparently a group of them, my forebears included, left Pennsylvania in the late 1600s, and settled in Georgia and the Carolinas, and some later in Alabama. That may explain why, in the late 1800s, two siblings of this family had married two siblings of a family known to be "mulatto" despite their mostly European appearance. One of the "Quakers" even put himself as "black" in the 1900 census (they took away the "mulatto" classification that year, though it was somewhat restored in 1910) when he was known to be from a white family. Yep. Self-congratulations by self-denial and identifying with the underclass was well underway at the dawn of the last century, thanks to Quakers who know what's best for us all.