Now, that having been said it is perfect proper to admit and consider slavery as a major and prominent issue during the war. To refuse it would be to deny history and engage in absurdity. But to do as McPherson, Marx, and other persons who advocate an historical view heavily skewered to the yankee side do and purport slavery to be the sole issue is similarly a violation of historical accuracy.
What you have posted is the official view of the South, designed to harness the emotions of whoever read it (propaganda doesn't have to be just for the masses). GOPcapitalist's article was an attempt to show the complexity of the Civil War, which goes beyond the propaganda both sides put out.
As for tariffs, the South denied them, felt their economy threatened by them. Thus, the North felt compelled to collect them by force. That is why Abraham Lincoln felt it was not invasion to collect money that was due to the federal government, as federal law supersedes state law. You're correct that once the South fired the first shot, the North could blockade without impunity.
To suggest that the war had _nothing_ to do with slavery is absurd, but so is to suggest it had everything to do with slavery and slavery alone. McPherson does the latter and that is my point of contention - he is engaging in historical inaccuracy. If you are interested in what the southerners themselves said I encourage you to look into a greater sample on the subject. As I noted earlier, only four states adopted non-binding declarations separate from their official acts of secession. The rest either did not formally state causes or included them in their statutory acts of secession, and about the only ones that discuss slavery as a cause in any substantial detail are those four non-binding resolutions.
I encourage you to check out the ordinances of secession. They were passed by South Carolina, Virginia, Georgia, North Carolina, Mississippi, Florida, Louisiana, Alabama, Texas, Arkansas, and Tennessee. Missouri's legislature passed one in exile after federal troops were dispatched to prevent them from doing so. Kentucky met in a convention and did the same. The territorial government of Arizona also passed one. So did the tribes of Indian Territory, with the most notable being the Cherokee declaration.
If you wish to gain a full picture of the situation you owe it to yourself to examine them all. The issues are very complex and scattered. Arizona, for example, complains of their mail services being neglected and their protection from frontier attacks having been denied by the northern government. The Cherokee declaration focuses on the violation of rights and liberties by the northern government. The Texas ordinance complains of the north's abuse of government power and violation of the social contract. Most of them are online and may be found with a simple search.
The blockade of the South was not about collecting tariffs.
Lincoln explicitly stated revenue collection was his justification for the blockade in his military orders instigating it. That was on April 19th.