I think you pretty much got the alternate history down pat. Many libertarians believe that had the Feds just backed off of the Confederates, the primitive agrarian slave economy would have been left in the dust of the industrialized North within a few years, and collapsed under its own inefficiency. A slaveowning farmer ends up looking pretty damn stupid compared to a farmer with a tractor. Slavery and the heavy manual field labor it involved was pretty much a dying concept even before the Civil War started.

Odds are that most slaves would have become excess baggage on Southern plantations as industrialization set in, and they would have been eventually freed, if you can call having the plantation owner boot them off his land as excess mouths to feed.

Maybe if emancipation had been a Southern idea instead of forced on them at gunpoint from the North, the bitter resentment towards blacks and the North that hung on for a century afterwards might not have happened. Who knows?

Of course, other schools of thought say the war was primarily economic and political in nature, and that the issue of slavery was a convenient hook to hang the war on for the benefit of observers and participants. A few libertarians feel that Lincoln was a major factor in the development of the overwhelming central power of government today, and feel less than kindly towards him.