"Since at least 90% of imports in 1860 entered through the North, it stands to reason that at least 90% of imports went to the North."
What makes you think they stayed there?
Why in the world would imports destined for, say Georgia, go overland from Boston rather than be delivered to the perfectly good ports in Georgia? Come one now, you are grasping at straws.
What they are looking for is a simple one line statement, analysis is beyod them. Please forgive those ignorant of economics, Warehousing Acts etc. Per their understanding, ships would sail from England to EVERY American port to deliver goods, and those without ports would never receive an import. They believe that it's economically cheaper to sail a thousand miles further to deliver goods, that foreign suppliers were not seeking quick turnarounds.
They also cannot comprehend the fact that imports dropped with the secessions, and products previously supplied by the southern states would now have to be imported causing imports to increase in 1862. They believe that that importers eat that tariff (in some cases up to 62% of the price), that the tariff is not passed on to the consumer, nor do they understand that tariffs REDUCE profits to foreign suppliers, and that due to the REDUCED profits, US exporters receive LESS revenue from those same foreign suppliers purchase American products.