In 1860, a majority of Americans had lived their entire lives within 100 miles of their birthplace.
The “Union” was simply a concept, and very few people, North or South, had personal experience with both cultures and their geography.
I can see that economic concerns about use of the Mississippi River would be very important to the North. And I can see that national security concerns about the redrawn borders of the USA would be very important, to both sides.
But those were issues that could be negotiated and, quite possibly, agreed on by both sides.
But I find it impossible to believe that, without the non-negotiable issue of slavery, the Civil War would have ever been fought.
Lincoln said the war was not about slavery. So believe it.
You overlook the fact that not only was the South paying between 50% and 80% of the entire Federal Budget, (Yes, the Federal Government was primarily funded by slavery) but by forming a separate country, the south horribly undercut Northern Monopolies in Shipping and Tarriff's, resulting in potentially billions of dollars in losses for Northern Interests, specifically New York and Boston. (Where most of the Wealthy and influential people lived.)
Pea Ridge has posted numerous period letters from Northern Interests wailing about their losses of revenue. (The one about New England Businessmen demanding Lincoln DO SOMETHING about their secession related financial catastrophes would be appropriate right about now.)
Of course no one wants to go down in history saying they killed 600,000 people to restore their lucrative businesses and monopolies, so they insisted that it was for a "moral" cause, though if that were their prime motivation, they could have outlawed slavery in the five Union states that still had it. They could have removed the "beam" in their own eye first.
The "problem" had completely ceased to exist.
On the 4th of February, 1861, the Confederate Congress, composed of delegates from the six southern states met at Montgomery, Alabama and completed their secession movement by adopting their own constitution.
This effectively ended the slavery problem for the Union states.
At this time there were 27 Union states, all with stable legislatures and courts. Their commerce was continuing and there was business as usual. In northern ports such as Boston and New York, ships were sailing their regular routes to Europe, and many continued their commerce with Southern ports.
Newspapers were printing, banks were lending, legislators were occupied, roads and canals were open, and the Federal government was operating.
It is likely that you find "it impossible to believe" because you have not learned of the influence of tariffs, Northern governors, and politicians' influence on Lincoln.