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Let's take these in rough order. There are a lot of moving parts here and I can always expand further if it's needed.

To start, the Civil War was obviously about slavery, but why is saying that it was (at least in part) about states' rights so wrong? How was the Civil War not a fight for a states' right to slavery?

Well, you said it right there. If the only state's right in contention is keeping slavery then the war has zero to do with state's rights. To do otherwise is actively misleading, since the statement will be parsed as "the war is about state's rights, which is somehow different from slavery" and the corollary "state's rights is somehow a cause in itself." These are equally absurd because no one believes in anything like state's rights. Rather state's rights is a slogan of protest one uses to object to some policy that the nation is undertaking, which the speaker intends to use state government power as a hedge against. In other words "state's rights" simply means "I lost the election and am resolved to reject the legitimacy of the national government until I win the next one." Or one might phrase it as "No one may govern except me," which is the actual position of the White South.

This was transparent to all involved at the time, just as it was when state's rights were mustered in defense of segregation by the direct ideological descendants of the same people. If anyone in the White South were in favor of state's rights as a thing in themselves -a logical impossibility, since they are not a policy but rather a means to secure policies preferred for other reasons- then we would expect the section to have voted as a bloc against the Fugitive Slave Act of 1850, when instead they were its main supporters. Said act overruled, with federal power, numerous state laws that recognized the federal obligation to render up enslaved people who stole themselves to freedom. Said people were a very popular cause in the free states, with even many whites who otherwise did not mind slavery finding the business of personally rounding up individuals and shipping them back into, or into for the first time since kidnapping was an obvious risk and reality, utterly distasteful. Most would just stand aside and do nothing if they learned about or suspected a self-stealing person was in their presence. Many might donate occasionally to public groups which aided these people. Very few would tolerate being drafted into slave catching, which is precisely what the Fugitive Slave Act required of them, whilst simultaneously stripping the accused of any means whatsoever to prove their innocence and making aiding and abetting them, or just standing aside and refusing to help ship someone back to a system of rape and torture for profit.

Yet this is what the Fugitive Slave Act required of them. It was, in essence, the nation's first conscription program. Every white person in earshot of a slave catcher could be deputized against their will and compelled under penalty of law to aid in the capture and rendition of any black person that the enslaver cared to claim was someone's property. This abrogation of personal liberty laws enacted by the free states to prevent or mitigate against literal kidnapping as well as the moral horror at seeing a person enslaved right before their own eyes, warranted nothing from the white South but furious applause. Nor did the white South rise against a Supreme Court ruling that, by any orthodox reading of what state's rights proponents claimed to believe, tyrannically rejected their entire philosophy of government by declaring that no state had the power to contravene federal law on this very subject.

When a distinct, identifiable group of political actors tell you their main objective is slavery's eternal perpetuation, act in a manner wholly consistent with that, and do not depart from that course in any substantial way for the entirety of their history even when said departure would square them better with their alleged philosophical and legal scruples, then it's no mystery at all what they're on about. To say they're after anything else is just lying about the past, the literal opposite of the historian's task. It is as wrong in American history as it would be for someone who studies German history to deny the Holocaust.

Moreover, how can we not call it the War of Northern Aggression when it was undeniably the North who provoked the war

It was not the North that rejected the outcome of an election fairly contested and fairly won, on terms to which it had happily assented in every prior occasion going back to George Washington. Nor was it the North that engaged in active rebellion against lawfully constituted government which had done it no harm and, indeed, had bent over backwards for its entire history to appease and facilitate its parochial concerns. The white South did not rebel, to the point of seizing federal property (including customs houses, mints, and arsenals) under threat of force and at literal gunpoint, prior to even losing its nearly absolute control of the federal government. For that act, you must look to South Carolina, Mississippi, and company. If winning an election is adequate grounds for rebellion, then the truth is that one has not assented to deciding who shall wield power by means of elections at all. That being the case would place the white South entirely at odds with contemporary American political theory, a difference which as it adheres rather precisely to the contours of enslaving, we may readily attribute to it.

So if you mean to suggest that the white South, being the chief exponent of authoritarianism in the United States, rejected ab initio the theory of government upon which the nation was based, and understood as such by contemporaries, I will not disagree for any era in which the white South is a recognizable political coalition. Should you also suggest that the vaunted rhetoric about a government of the people, by the people, and for the people is a lie on the grounds that "the people" means "white men, then engaged simultaneously in enslaving one population and genociding another" I again cannot disagree. Few historians could, if we're even a little bit honest.

But it's quite a reach to argue that the white North, after having enslaving increasingly forced upon them by an unrepentant and authoritarian South, to the point where the practical viability of having even a single free state was increasingly in question, somehow provoked the white South by electing politicians -and not even a congressional majority of them!- dedicated to reversing that tide constitutes some kind of provocation for which armed rebellion is the proper cure. To do so is simply to argue that antislavery politics, no matter how moderate and inclined to grant numerous concessions right up through 1861 simply has no permissible place in the body politic and must be actively suppressed, a project which more radical leaders in the white South had adopted decades prior and were still sore about not getting right up into December of 1860, when South Carolina complains that the free state haven't repressed their antislavery groups into oblivion. In other words, one must argue that objecting to slavery is morally on the same level with advocating genocidal violence, which is what the white South actually believed.