Similarly, no matter how much Southerners complained that they were not getting their fair share of the money raised by those tariffs which they as the exporter/importers were paying the overwhelming share of, for things like infrastructure projects, corporate subsidies, subsidies to certain industries, light houses, coastal forts, dredging harbors, etc etc they once again could not say it was unconstitutional. There is nothing in the US Constitution that requires the federal government to apportion its expenditures fairly or evenly.
Slavery however gave Southerners a legitimate constitutional argument. Northern states had refused to enforce the fugitive slave clause of the US Constitution and had gone considerably out of their way to impede the recapture of escaped slaves. This was blatantly unconstitutional. Here in this issue, Southerners had constitutional/legal grounds to say the Northern states had violated the Constitution and that therefore the contract was broken and they were out.
Lincoln orchestrated and the Northern dominated US Congress immediately passed the Corwin Amendment - which Lincoln endorsed in his all important first inaugural address - which would have explicitly protected slavery in the US Constitution effectively forever. Protecting slavery was the very first bargaining chip they were prepared to offer up if only their cash cows - the Southern states - would come back in. The original 7 seceding states said no. They were not interested in slavery forever. They wanted their independence.
Both sides knew the Southern states would be much better off and the Northern states would be much worse off if the Southern states became independent.
And Shemrman ended that argument.
“which would have explicitly protected slavery in the US Constitution effectively forever.”
Not really. All the Corwin Amendment did was to prevent the U.S. Government from taking any action against slavery in any state where it was already legal. States were free to end the institution if the so chose to do so, and Congress had the authority to prevent the introduction of slavery into any territory they so chose.
It was only protecting slavery in the states where it already existed. It did not protect slavery in future states. It needed 3/4 of the states to ratify it, which seemed unlikely.
The Southern aristocracy was all about expanding slavery into new territory. It was the only way they could figure to keep the institution alive. Cotton needed new ground.
Here is the proposed Corwin Constitutional amendment:
“No amendment shall be made to the Constitution which will authorize or give to Congress the power to abolish or interfere, within any State, with the domestic institutions thereof, including that of persons held to labor or service by the laws of said State.”
FWIW -- I think your whole post is very well expressed and will take considerable effort to unpackage and correct.
So, others have already pointed to a flaw or two in your logic, but there is much more to mention.