There are newspaper editorials of the day which point out that it would have been impossible to regulate the border between the CSA and the USA. The goods would have gotten through anyway, and the Mississippi would have carried them deep into the heart of the country. New York and New England would have been twice hit. First in the loss of their shipping, handling, and banking industries because they would be bypassed, and secondly in the loss of manufacturing business because their products would have been replaced with cheaper and better quality European goods.
It was a double whammy, and do not think the industrialists of that era were stupid. They knew exactly what sort of threat an independent south would pose to their economic interests.
I believe the Maritime Charleston website quoted words to the effect that New York's economic hinterland stretched to Kentucky and beyond, while Charleston's barely reached East Tennessee, if that.
Most traffic shipped out of New Orleans. Charleston was a more local market, but still one that would displace northern products and services.
New Orleans is where the real trouble was.
If the Confederates took over Kentucky things might have been different with smugglers crossing the Ohio, and that may be why the CSA was so determined to seize Kentucky, but you are assuming that the difference in cost and quality between US and foreign goods would be enough to make up for the greater transportation cost and the risks of smuggling. That is a very questionable assumption.
Also, those smugglers would also be carrying runaways to the North, thus weakening the slave economy. The river border would be so thick with troops, customs officials, smugglers, bushwackers, bandits, abolitionists and seditionists that the likely result would have been war whatever happened elsewhere.
Buchanan's Secretary of War was a traitor who did all he could to help the secessionists and weaken the country's ability to respond to their provocations. He resigned at the end of 1860 because he disagreed with Buchanan's unwillingness to surrender the fort. You assume that the secessionists were so well informed of what was going in Washington in the early days of the Lincoln administration. Do you really think that they weren't aware that John Floyd no longer spoke for the Buchanan administration and that his treasonous collaboration could no longer be assumed to be federal policy?
Why didn't we go to war with Brazil? Because we weren't idiots or busy-bodies. Because Brazil didn't attack our fort and our flag and threaten our capital and our fellow citizens.