Sleight of hand. Whether it was economically feasible or not wasn't the point, control was. The secessionists are quite clear, you just have to read their own words. Control of the west was the key sticking factor, which is why they could not accept any compromise from Lincoln.
Exactly! The Northern coalition had control of congress, and any attempts to add "slave" states to the Union would threaten that control! The "Navigation act of 1817" could be repealed, and that would have seriously damaged Northern shipping industry, the "Warehousing act of 1846" could have been repealed, and that would have damaged New York port value, the Tariffs could have been lowered and the Southern states could have purchased more inexpensive European products.
There was a *LOT* of money riding on the control of Congress, and slavery was just a proxy fight for this control.