So then you're saying the whole reason for separation was slavery?
The Vast bulk of European trade went through New York.
Imports yes. But the vast bulk of Southern exports, which made up the large majority of U.S. exports as a whole, left from New Orleans, Mobile, Savannah, and Charleston.
In addition, laws were in effect to more or less force trade to go through New York. Foreign ships were not permitted to carry goods between American ports. Only American ships were permitted to do that.
True, just like only British ships could carry goods between British ports. But that restriction did not apply to ships bringing goods into the U.S. from abroad, or ships taking goods from the U.S. directly to foreign ports.
Point of trivia, that law is still in effect. Which is why if you take a cruise from Los Angeles or San Diego to Hawaii and back the ship makes a brief stop in Mexico.
New Orleans and Mobile were busier because people HAD to go to these ports to get Cotton/Agriculture Shipments from them. Transporting these cargoes overland wasn't practical, so those ports had to be used to access that territory's products.
They were busier because they were the most convenient port to export cotton and other agricultural products from.
Going independent would allow foreign trade ships to stop in New York, and then go on to Charleston, and the packet shipping, (which mostly benefited New York) would have taken less of a cut of the trade traffic.
Nothing stopped foreign ships to go directly to Charleston or New Orleans with imports prior to the separation.
Also the reduced import duties that would have been available from Charleston would have boosted traffic there dramatically.
That makes no sense at all. If they were separate countries then what difference would Confederate tariff rates have on U.S. imports?
An Independent South would have made Charleston (and surrounding area) a far more wealthy city, and it would have come at the expense of New York and to a lesser extent Boston and Philadelphia.
And I don't think you make your case at all. In an independent Confederacy the same exports that left from Mobile and New Orleans would continue to leave from there; Charleston wouldn't take from them so if any cities expanded and grew more important in an independent South it would likely be those two. And imports destined for the North would continue to flow through Boston, New York and Boston. Separation wouldn't change that.
Your comment about Mobile and New Orleans raises an interesting point. I’ve always assumed that the growth of commerce on the Mississippi River system played a much bigger role in the Civil War than most people realize. The U.S. could not have expanded the way it did if the Union states in the Midwest and (later) the Great Plains could not transport freight freely up and down the Mississippi River.