Like most lost causers, you're taking advantage of the war's assymetry to deny the centrality of slavery as a motivation for secession.
Let's start with South Carolina's Ordinance of Secession. Frequent references to slavery, & the hostility to it by the nothern states.
Then there's VP Alexander Stephens's infamous "Foundation Speech" (which one lost causer referred to as "hyperbole", hunh...?).
Slavery motivated secession. Secession motivated the north to go to the war. So you're right, as far as it goes, the kid from New Hampshire wasn't dying to free slaves; he was dying to preserve the Union. Sure, some Union troops were abolitionists, & many were racist. In fact, it makes perfect sense for someone who hates Black people to be an abolitionist, because slavery, after all, was the reason most Black people had come to the US.
Why would some kid from Alabama die to preserve slavery? Well, according to the digital history project, about one third of all southern families owned slaves.
But, even if he didn't, suppose, by comparison, a foreign power could cut off US petroleum supply tomorrow. That would destroy the US economy completely. It would an authentic causus belli, with the vast majority of troops in no way connected to the petroleum industry prior to hostilities.
Of course Lincoln spoke of not inteferring with slavery, because he wanted to avoid southern secession, which he knew would lead to war.
The Emancipation Proclamation freed no slaves, true. Lincoln needed to keep the border states on the Union side, which meant not interfering with their slaves, while also keeping Europe out. It was a pragmatic, wartime measure. His shepherding of the 13th amendment banning slavery spoke volumes, however.
Were the northern armies brutal? At times, sure. Sherman set out to make "Georgia (& South Carolina) howl", & he largely succeeded. Neo-Confederates complain bitterly about their treatment at the hands of the north, which is ironic when you consider that the Confederate leadership doubtless viewed themselves as latter-day Founding Fathers, & what did Ben Franklin say about hanging together or hang separately? For all that Confederate leadership wasn't punished, by & large.
You didn't burst any bubbles here. Yes, the north fought to preseve the Union, not to end slavery. I'm always amused by the notion that a successful Confederacy could somehow remain intact. With secession as the solution to states' conflicts, I have no doubt you'd see Balkanization in both the north & south. Why, for example, would Florida & Texas remain in a union with the Deep South? Why would California & the other western states remain in union with the remnant of the north?
Because Lincoln preserved the Union & thus the United States, I think we'll keep the memorial for a while longer....