"The Morrill Tariff was THE ISSUE."
Not in January 1861, since the Morrill Tariff was not finally approved until March 1861, and took effect in April, after Civil War had already begun, at Fort Sumter.
Second, the figure "70%" is wildly inaccurate, since of the US top ten ocean-ports, accounting for nearly 100% of dutiable imports, only number six, New Orleans, and number ten, Charleston, SC, seceded.
Based on city populations alone, these two Southern ports cannot have accounted for more than 10% of all US dutiable imports.
Yes, later, war fears along with Confederates' embargo on exports of cotton, might put a damper on some commercial activities, which in turn could affect the US treasury -- in March or April, 1861.
But not in January and nothing to do with the Morrill Tariff, which was not approved until March 2, 1861, took effect in April, after Civil War had begun.
PeaRidge quoting somebody unnamed: "Over one hundred leading commercial importers in New York, as well as a similar group in Boston, informed the US collectors of customs they would not pay duties on imported goods unless those same duties were also collected at Southern ports.
Please provide a source for this data.
It sounds like businessmen threatening law enforcement with non-compliance, a potential problem for which law enforcers have tools readily available.
PeaRidge: "This threat was likely the proximate cause of the beginning of the war.
The Lincoln Cabinet abandoned its initial inclination to turn over Ft. Sumter to the Confederates, and to support Lincoln's plan to invade Pensacola and Charleston."
First, not "likely" if you remember everything else going on in those days.
Second, in March 1861, Lincoln had no plans to "invade" anything.
The question then was whether to resupply and or reinforce Union garrisons in the two remaining Union forts in Charleston and Pensacola?
Lincoln felt it his duty as president to defend Federal properties, but was willing to trade Fort Sumter for a promise by Virginia not to secede.
Third, when Virginians refused to make such a promise, Lincoln decided to resupply Fort Sumter, and so advised South Carolina's Governor, Francis W. Pickens.
Pickens immediately informed Confederate President Davis, who ordered Sumter be assaulted and taken by military force.
Davis' order was not the "likely" proximate cause, it was the certain beginning of actual military warfare.
You missed the point. As of January, no products from the seceded states entered the Transatlantic trade, nor were transshipped to northeastern ports.
Since Southern products accounted for 70% of the trade inventory, this was lost.
This caused a great stir in New York and New England.
Read the newspaper accounts published previously.