Everything you wrote made sense. I don’t get this idea that the Navigation Act of 1817 was part of some Yankee conspiracy to enslave the South. I’m doing a lot of research and it reflects part of what you alluded to. Companies weren’t all “Southern” or a “Northern” or even “British.” There was a whole lot of synergy going on, just like today.
The key fact here is that DiogenesLamp hopes to impose his own historical narrative on historical people who would not recognize it.
He claims they were being forced by law to use certain shippers or certain companies -- "Northeastern power brokers" -- when the fact is they weren't "forced" to do anything, but instead did what seemed at the time the best things to do.
Today the US has dozens of large ports, including inland ports like St. Louis (#19).
Of those, New York-New Jersey is number three overall, behind Houston and New Orleans.
New York at #3 handles roughly 5% of total US freight tonnage.
So there is nothing unique or monopolistic about New York today, nor was there in 1860.
The Warehousing Act of 1846 was likewise passed by a Democrat Congress and signed by a slaveowning president, James K. Polk. That was the Congress that passed the low tariff of 1846 and the law was intended as a free trade measure. As with the Navigation Act, it was only later that some Southerners started to see all this as part of a Yankee plot.
The idea that the Navigation Act meant that Northern shippers set rates only a little higher than the fines imposed on foreign shippers has some things wrong with it.
First of all, there wasn't a cartel that could enforce rates, so far as I know. US shipping companies were in competition with each other, and if the rates were too high, planters and merchants could ship directly to the Old World on British ships, since the Navigation Act didn't apply to shipping to and from Europe, so there was a limit to how high rates could go.
Secondly, why would using British ships automatically be cheaper? You could make the case the established British millowners still produced thread and cloth that was better or cheaper than US products, but why would that apply to shipping, something that Yankees were as experienced at as were the Britons?
Third, the idea that British companies would be shipping between US ports and incurring fines is a little sketchy. If you're not welcome and there are fines and prosecution involved you go into business somewhere else.
The law effectively kept the British out of US domestic shipping. Prices may have been a little higher, but developing US shipping was considered to be worth it. If the US abolished the law it wouldn't mean a flourishing of Southern-owned shipping, it would just have meant that British firms would enter into competition with US ones.