But not much stronger until the cotton gin came along, and more importantly, after Nat Turner's rebellion, which helped kickstart the awful trend of slavery becoming not just a way of business, but a way of life, and as part of a means of social control.
Well, the first effect of Nat Turner's rebellion was near-consensus in favor of emancipation in Virginia. Once that solution had been turned down, though, yes.
Not sure what you mean by "not much stronger until the cotton gin" given how quickly after the Treaty of Paris it was invented. It appeared before the recovery from the war.
Perhaps possible, but dubious north of say, Maryland, barring significant changes in culture(which isn't at all likely).
Happened in OTL, even in Pennsylvania, even over a generation into the process of gradual emancipation. Ironworks in Scotch-Irish PA employed slaves wherever possible. The money was just too good.
Despite slaves being less efficient than free workers, per capita? Southern factory bosses may have favored them, because slaves didn't have to be paid a wage(amongst other things), but industrial slavery enjoying even modest success anywhere in the North, would be very hard to do, and damn near impossible east of Pennsylvania: not the least of which that many white workers would not stand for being replaced by wage-free labor, black or otherwise.
It doesn't matter how efficient a freed worker is, if he decides to move to a better job or a home closer to family. Which they did constantly, because industrial work was low status. That's when the slaves paid off, and overwhelmingly so.
And again, it was in fact a modest success in the north historically.
That said, no doubt any gains by slavery would be temporary. So in that sense we agree. You are quite right about the feelings of free workers, and it's an area of the country with a steady supply of immigrants who were willing to do the work for low pay. The risk isn't slave NY in 1890, but of the gradual emancipation bills in those two states being delayed for years, even over a decade, by vested interests.
Which would very likely be shot down, in either state, by not just Yankees in general, but even Southern transplants not necessarily friendly to slavery, especially if it's at any time before 1840, and especially in Illinois.
Almost happened in OTL in Illinois. Barring the fortuitous election of a governor who was anti-slavery (but would not have been elected had slavery been an election issue) Illinois would most likely have become a slave state shortly after statehood. This was because there were already quite a number of slaves in the territory before statehood and the general consensus was that it was a fait accompli.
I find your "especially if it's at any time before 1840" completely mystifying, as the growing anti-slavery feeling in the Old Northwest is common knowledge and well documented. The only time it would have been possible was shortly after statehood, because that was - obviously - both a state and the place of residence of active slavers. In what sense would you see the odds of slavery's introduction increasing after 1840?
As for "especially in Illinois," it is contraindicated by the facts.
See above.
"There are more things in Heaven and Earth, Horatio...."