How could the US civil war have been avoided?

It need not have happened at all.  This also makes clear that cotton drove the expansion of slavery itself.  After all, there was always an alternative to chattel slavery in the form of indenture which had by then evolved to a seven year contract.

Slavery became nonviable with the advent of fiat money as it provided an alternative source of capital besides individual labor.  Thus the first half of the nineteenth century was its last hurrah even globally with the british forcing the issue globally.

It never really made good sense in agriculture, not least because you had forced periods of near idleness for any crop based source of income.  Labor as capital only makes sense with continous employment as in the case of a house servent.

Ag always depended on surge labor and the rest was maintenance readily handled by the owner on family farms.  Even a field of cotton cannot be picked in the offseason.

augmented by owner operated horsepower, we have begun transiting to rotation and multi cropping.  Even harvest labor can be built around at least six months of work.

How could the US civil war have been avoided?

Make the compromise of 1792 work, instead of fail.

In 1792, Kentucky joined the Union.

Its larger planters had slaves, and wanted to keep them, in spite of the strong sentiment still running for freedom in much of the country, 9 years after the Revolutionary War ended. The compromise proposed was that Kentucky be admitted as a slave state, but, in the same law that admitted Kentucky, would come the statement that no other applications for slave states would be accepted. It failed in the US Senate.

It had succeeded overwhelmingly in the House, but the Senate was so close that Virginia’s Senators were the deciding votes. Virginia, even then, had soil so drained of nutrients for tobacco that their slaves, to be sold to other planters, were their most profitable item for sale. That meant their politically dominating large plantation owners were against the bill, even though the western Virginia farmers, who had far fewer slaves, on smaller farms in the hills, were for it.

It failed. The compromise was removed, and Kentucky joined, anyway. The slave-state confederacy grew from that point, and the invention of the Cotton Gin, in all its ugliness.

If the New England states, and Central States had been willing to make the further compromise, that slaves sold into the western territories would be legally converted to indentured servants under a legal arrangement for the western territories, and those indentures sold in new States after Kentucky joined, then the Slave Confederacy could still be aborted after 1808, when imports of slaves became illegal, because a person could not be bound for more than the 7 years after their slavery was converted to indenture.

That would leave Kentucky and the coastal slave states as the only places where slavery, rather than indenture, was legal. Since importing slaves was illegal after 1808, and all sales to western territories were indentures, then the slave states would be isolated on the East Coast as the number of slaves dwindled. There would be no fantasy of uniting slave states between the East Coast and the Mississippi, much less Texas.

The 1792 slave states would have been slowly drained of both their slaves, and of their oligarchs’ determination of reducing to dependence on large plantation owners both the free and unfree sections of those states’ populations. By 40 years after 1792, when the ideas of slavery domination were growing, we would, instead, have a deeply weakened ability to sustain slavery at all. I doubt slavery would have survived the 1830s. The use of slave possession to denote class status would die with it.

The results, on the Republic and its economy, would have been mind-boggling over the 100 years between 1792 and 1892. With small farmers dominating the legislatures of the South, the Western States and the Southern States would unite in opposition to tariffs, moving the US toward free trade far faster. The lack of plantation oligarchs to dominate the labor markets of the South, as happened after the 1877 Disaster, would mean that there would be no more opposition to industrialization there than in the Midwest. Instead of Southern Industry not getting started till WW2, it would likely begin by the 1820s. The US would not only avoid the Civil War, but most of the anti-industrial romanticism that we experienced after the Civil War, to the present day.