On the whole, I think Americans might well have been willing to let the Southern states go, but not on their own terms, and once violence began, it wouldn't be easy to put things on a peaceful footing again. In other words it wasn't the idea of Southern independence that was regarded as frightening or threatening. Most Northerners probably would have accepted that, but they didn't accept way the rebel states or the CSA conducted themselves.
Most Northerners probably didn't have an objection to dissolving their union with the slave states, but they would have preferred a slower process of separation conducted through national constitutional institutions like Congress. Probably Northerners would have accepted unilateral secession if its leaders had been less provocative and more deferential to Northern concerns. The Confederates weren't interested in such a course. The Sumter crisis was their probationary period and they didn't pass it.
The secessionist leaders apparently wanted to crow about the new unrestricted freedom of action of their government and to put it to use immediately. A more modest, restrained, ... and well humble ... approach that moved slowly and paid a certain tribute to the nation and institutions that were being abandoned would have had better results. It was precisely how the secessionists behaved that got them into trouble, not the desire for independence itself. Do you deny it?
My point was to offer an explanation of why things happened as they did, and why the secessionists' actions were bound to bring about war. Yours seems to be to defend the actions that produced the war and, apparently to argue that they could have or ought to have had a different effect. I don't know how one could ever prove such a thing. It seems to be a rather dubious proceeding -- to cut out all the actions, ideas and emotions that produced a given result and argue that the result should have been different.
I stand by my comparison to more recent wars. It wasn't wholly intended as a condemnation of Davis, more as an indication of the way history, especially American history works. Rouse passions by starting a war, and we will make you pay. Reason or mercy may dictate half-measures and compromise, but once the fighting starts, we don't take them.
As with all things political, widespread apathy will be the majority response to almost any event. A handful of partisans will tie the issue to something that people care about, or at least pretend to care about, and then things become heated. Probably so with the case of Southern secession, wherein both sides of the split sought an event which would bring their respective nation into a rash of patriotic fervor whether war came or not. You can point to commerce raiders or confiscation of property, but I could point to refusal by Union leaders to even attempt working a peaceful solution.
Northerner's may well have preferred that the Southern states showed more respect for the Union and tradition which the left behind, but what more humiliating thing can one ask of a free man than to bend to political power and beg for his freedom from the very hand of tyrrany which he views as such a threat? In this respect, I've always found Lincoln's penchant for graciousness after destroying the South one of the great ironies of American history.