That all those goods would land in southern ports, get taxed, and then get smuggled into the U.S. in dribs and drabs without exceeding the cost of the U.S. tariff? That's what passes for logic in your world?
30% price differential is quite a lot in an era where people struggled to make money. People smuggle cigarettes now. Perhaps you are unfamiliar with this phenomena?
So no, not so far fetched as you are trying to portray. 30% is just the profit differential from TAXES. Additionally there would be profit differential from the European goods being cheaper, and of better quality, so profit potential for a smuggler could be substantially more than 30%.
And people smuggle crap in and out of Canada every single day. Canadians often come to the US for healthcare, and take it back with them when they return.
Not at all unreasonable to believe markets would be set up south of the border, and Northern citizens would avail themselves of these markets. They certainly couldn't have had border outposts all along the border, and there was always the chance that Kentucky, Missouri, etc would decide to avoid the hassle and just join the Confederacy.
That's what passes for logic in your world?
It passes for logic in any reasonable world.