I'm sure they could have, but if such usages were profitable, why didn't they? I don't know too many people who willingly leave money on the table. Most people try to maximize their investments.
You're not entirely wrong. Slavery needed a highly profitable application to be politically secure. That was cotton.
But once slavery was established, slaves would be used for a variety of other purposes. The fact that cotton-growing slave plantations were profitable didn't mean that tobacco or hemp or indigo or rice or sugar plantations didn't make profits working slaves. Plantation owners didn't just give up their slaves if they couldn't grow cotton.
In the slave states, small farmers used slaves to grow cotton and rise livestock. Slaves were also employed in homes and shops. They worked in early textile mills and iron foundries. Slaves worked in lead mines in and also in Illinois where slavery was illegal. Slaves provided most of the labor for Southern railroad construction. The fact that cotton growing was profitable didn't mean that those business also didn't make money. And there were enough slaves that not all of them could have worked in the cotton fields.
So it was to be expected that if slavery were politically secure, slaveowners would move to new areas and bring their slaves with them. When slavery was socially and culturally acceptable and politically secure, slaves were used in all kinds of businesses. Ranchers probably wouldn't want to rely on slaves, since it would be too hard to keep them from running away, but mine owners wouldn't have that problem.
Your big idea a few weeks back was that the Confederacy would be a roaring economic success, even to the point where Midwestern states would be tempted to join the CSA. That's nonsense, of course, but assume it were true. If the slave system was such a success, why wouldn't other states want to take it up?
Machinery or Machined goods from Europe had high tariff's slapped on them to make the Northern produced products competitive. With the tariffs, it was cheaper for the Southerners to buy the Northern produced goods. Without the tariffs, it was cheaper to buy the European goods.
Well, there too, you're not 100% wrong. Tariffs often help keep inefficient or second-rate manufacturers in business. But in the late 19th and early 20th century, tariff-protected US and German manufactured products were usually far superior to what the free trade British turned out.
Domestic competition drove US industries to improve their products while British innovation languished. I wouldn't assume that British products were always superior to American products even in the antebellum period.
The stereotype of the British Victorian Age is that it was always grim and bleak. That wasn't entirely true, but I wouldn't go to the other extreme and ignore that there were good reasons why we think of Victorian Britain in that "Dickensian" way. British industry succeeded in a "brute force" kind of way and this left the British less inclined to explore new methods of production and sales.