I've read this book, not sure how you think Watson would have a problem with it. Nelson just looks at all sorts of decisons and how they could have possibly avoided the war being done differently. Using that premise, if the south hadn't overreacted thinking Lincoln would free their slaves and restrict expansion into the territories when he clearly said he would not they never would have seceded.
Sure if Lincoln had just bid the south good luck the war could have been avoided but then the Unitied States would be little better than the Balkans on the world scene. That is certainly a what-if that could have avoided war but made the resulting nations a collection of weaklings.
If you are hanging your hat on Lincoln ignoring his cabinet it was no different than Jefferson Davis ignoring his cabinet and explicitly Robert Toombs whose advice about firing on Ft.Sumter was, "Mr. President, at this time it is suicide, murder, and will lose us every friend in the north." Lincoln's relief expedition had specific orders not to fire on Confederate forces unless fired upon them first.
Lincoln had the right to defend US military installations, if Hawaii had joined the E#mpire of Japan in 1941 would the US have been obligated to give up Pearl Harbor, Schofield Barracks and other military facilities?
As Andrew Jackson said during the Nullification crisis the Constitution did not form a league, if any state had a right to pull out then the US was not a nation.