Why would they have to sail with an empty hold? I posted numerous examples of foreign ships bringing goods with them to Southern ports, all taken from contemporary newspapers. Furthermore, foreign ships could bring passengers from Europe and return with goods from America.
To recap:
The reason for secession was the South’s desire to preserve slavery.
The trigger for secession was the election of Abraham Lincoln, which fed fear of the abolitionist foundation of the Republican Party.
The American Civil War began when units of the South Carolina Militia fired upon a Union fort in Charleston harbor.
You are trying to step around the fact that protectionism works. If there is no difference, why pass a protectionism act?