Hummel's argument is based on hindsight, not on what reasonable and informed people had reason to believe at the time on the basis of the available evidence. It's one of the tricks historical writers play on the dead.
Whatever the reasons why war began, by its end it was certainly, whatever else it was, also a war to free the slaves, and Lincoln did much to make it so. It was Seward who brought up the pending 13th Amendment at Hampton Roads and said that the Southern states, if they reentered the union would be able to vote against it. That was only what was true. If the secessionists rejoined the union, they would be able to have some voice in the affairs of he nation. Hay and Nicolay say that Stephens and the other Confederates made more of this than the Unionists did.
But Lincoln made it clear that Emancipation was always his policy. Lincoln who made it clear that slavery was doomed. According to Hay and Nicolay, Lincoln wasn't a party to Seward's way of persuading the rebels, and expected the 13th Amendment to pass, with Southern support. That's one reason why peace efforts failed at Hampton Roads and on other occasions. Stephens by contrast, wanted war with the Mexican regime, as a way of getting cooperation between the two sections without giving up the Confederacy.
You apparently want to make the war into a conflict between the compromising Lincoln and the rebels who stood firm for their "freedom" and "independence." But observers, then or now, have to consider just what independence would have meant for the slave states and what the Confederacy would have done with it. It's not balanced to judge Lincoln on the practical means he undertook to pursue his ends and not consider the practical policies that the Confederates adopted or were likely to adopt.
Don't be deceived by the "everyone believed in secession before Lincoln came along argument." It's not true. Many, if not most Americans, believed unilateral secession to be unconstitutional -- a form of revolt or revolution that could only be justified as a rebellion against real tyranny and repression.
A state could still turn to Congress or the constitutional amendment process to win approval for its separation from the union, but for a state simply to declare its relationship with the union dissolved wouldn't have been accepted by many Americans as constitutional.