NapoleonXIV said:
What if the planters had built their own textile plants?
They did, to an extent. If you mentally divide the South and North into separate nations, the South was the sixth-largest producer of cotton textiles in the world in 1860. Britain was far and away the largest, with the North coming a distant second.
Would the tariff had been such an issue?
Yes. The South had difficulty competing for exports of textiles with Britain (see below), and thus producing more textiles wouldn't have really helped. The objection to the tariff was that it meant that Britain could have retaliated with tariffs on the other manufactured goods which the South wanted to import.
The South's textile industry (and the North's for that matter) couldn't compete with Britain's for _export_ purposes, in part due to Britain's more efficient manufacturing industry and in part because of transportation costs - British shipping was more efficient and also closer to the prime markets (i.e. Europe). The North's and the South's textile industry, by and large, served only the domestic market (and a bit in Canada, if memory serves).
Would they find slavery inefficient for factory workers and begin manumitting a pool of workers?
No. Slaves worked just fine in factories. They weren't used more in factories because cotton was profitable enough that it sucked slave labour out of Southern manufacturing. But then, it sucked it out of other agricultural crops too. The use of slaves in tobacco and rice was declining largely because cotton was even more profitable.
Would the CW be different if the South had the robust metal and machinery industries that the North had developed largely to serve the textile base?
The South did actually have a surprising amount of industry by world standards. If they'd had a bit more, it might have prolonged the war a bit, but the weight of manpower and industrial power would still likely have rested with the North.
Would it have happened at all?
Probably. I can see ways to avoid the ACW, but increased Southern manufacturing doesn't directly lead to it. Barring butterflies, of course.