> Free men with tractors were about to tear out the foundations of slavery.
Slaves can build tractors.
> We know that slavery was economically infeasible by 1920
This was a generation after it was abolished and the South had had to do something else. It's my contention that had the South been able to hang on to slavery, it would have found ways to work it into niches it never filled in the real world. Assembly line labor, for example, or mining, or fishing.
> the CSA let the abolitionists choose the terms of the conflict.
The abolitionists, by and large, thought they could win enough seats in Congress to outlaw slavery if they could prevent slavery from expanding into the new Western territories. That's why such expansion was the main topic of the Lincoln-Douglas debates, and why Bloody Kansas was so bloody, and so on. There were certainly militant abolitionists, but they were the minority; the CSA chose its own war.
> In a Christian nation, the master is responsible for his estate's dependents, even if there is no use for them.
In a Christian nation, slavery either doesn't exist or is infinitely gentler than what actually occurred.
> After farm automation, most plantation owners would have freed them in desperation to escape the room and board costs.
Farm automation means machinery, and slaves can make machinery. Maybe that would involve plantations being turned into factories, but it isn't a slam-dunk that it would involve freeing slaves.
> It has been argued that Reconstruction had to wait for the Manhattan Project and the Apollo program, which diverted vast amounts of resources into Southern industrialization.
This is an interesting idea. Who argues that?