Perhaps that is what they should have done, but trying to second guess why they didn't do it does not touch on my primary point that they were seeking to remedy the problem by eliminating all existing legal restrictions on their doing business directly with Europe, and that by doing so, they were effectively cutting New York (mostly) out of a very lucrative income stream.
That is my primary point.
1. New York was going to be cut out of a very lucrative income stream that they had enjoyed for years, and that had helped to fuel their economic boom.
My additional points are:
2.Not only would New York lose this lucrative income stream, they would suffer competition with their existing industries from competitive European products that would flood both the Southern and Midwestern markets through the Southern ports, thereby depriving them of additional income because of lost sales.
3. This supply stream of lower cost products from Europe through Southern suppliers would acquire patrons from the Midwestern states (who would trade their products such as grains, cattle) through the Southern suppliers, thereby bypassing Chicago and New York.
4. This set of increased economic ties between the new states/territories would result in a strengthening of the political coalition of the Southern Confederacy, and over time, these states would have become members of it rather than part of the USA.
5. The border states (Delaware, Maryland, Kentucky, Missouri) would have quickly joined the Confederacy once the money and products started streaming through Southern ports, making the ratio of states 15/18 Confederate/Union, instead of 11/22, and that would have been effectively impossible nut to crack with the remaining forces of the Union.
The political and economic ties over time would start to look like this, which is it's natural affinity.
The Free Trade policies (closer to) of the Southern Confederacy was a grave economic threat to many Northern industries.
Now, there were no existing legal restrictions on their doing business directly with Europe. They were perfectly able to do what Northern businesses were doing, they just didn't do it. At least not when it came to the shipping and handling aspects. They also chose not to build many textile mills, so that domestic production was centered up north.
1. The fact that trade did not fall as much as you claim it should have after 1861 shows that while the Confederates may have THOUGHT that New York was going to be cut out of a very lucrative income stream, in fact they were not. Then again the whole Southron myth of King Cotton proved to be wishful thinking.
On the other hand, assuming your speculative alternate history was based on a CSA that was allowed to peacefully secede, you have other issues to deal with. Where did the capital come from to build new ships, railroads, warehouses, insurance firms, etc. to handle this new aspect of the market? Crops like cotton sent to New England mills now became imported materials. How quickly would the Yankees look for other suppliers from other parts of the world?
2. Wasn't it you who said that exports and imports were roughly equal? So foreign imports were ALREADY coming into the country. If more came in then yes, domestic manufacturing would have likely suffered. On the other hand import businesses would have made money. But nothing was stopping people in the South from dealing directly with Europe anyway.
3. Maybe there would have been a steady stream, and maybe there wouldn't have. Maybe American manufacturers would have devised efficiencies that would allow competition on price. Now you're the one second guessing.
4. The West and South were already generally allied as free trade advocates. I don't see how Western states and territories would somehow join with the CSA over the USA of which they'd have remained a part.
5. I think that's rank speculation.
Your problem is US political ties could never look like this so long as the Confederacy remained the world's last best hope for slavery.
Also your "let them go in peace" scenario, where the Confederacy somehow consumes 1/3 more of the Union without war assumes, in effect, a President Buchanan type victor in 1860, but then there would be no need for secession.
Possible map of slave-power victory in Civil War: