The only place I reckon you ain't never been is Ft. Smith, Judge.
And what your mischaracterize as a straw man, is merely the direction of a curve of reasoning, only the asymptote of which is the "perfection" of the North having absolutely pure motives. No one need inhabit this position for it to exist for it to exist as a vector (i.e., a line of reasoning along which apologists may postulate varying degrees of Northern "pureness" in the cause of the war). The most extreme version of this narrative can be found in the school lessons of children. More nuanced versions merely inflect "realities" of slavery as a majority influence. All positions following this line fall along the same continuum.
As for proximate and ultimate causes, the only ultimate cause is the hand which moved the big bang and set the Leonids streaking in night sky. What is a cause or an effect in any given analysis, largely depends on what one is interested in and where one cuts off a temporal sequence. One might argue that the slave trade started the war, or cotton, or human nature. Much depends on perspective.
Were I, however, to note a rule of reason which would indict the North as the actor and aggressor, and thus the cause, I would cite that the South, in seceding had the defender's advantage. They weren't trying to invade the North. They were only trying to leave a bad relationship. It was the North, like a jealous husband, who would not let the lady South go. No invasion by the North, no war. The South, thus enjoyed the moral advantage of the defender. You, Sir, would bismirch the honorable lady South by suggesting that she "caused" her own beating by attempting to withdraw from her abuser! And in this case, the North would indeed have to defend it's motives in bringing war to peaceful states which voluntarily entered a union of states and voluntarily attempted to leave. If the North prosecuted its war in jealous rage, the nearly half-million is a high price to pay for the insistence that marriage is forever.
On the other hand, if it were a war to end slavery, the blood sacrifice would cover sin of an even greater evil.