If Congress has plenary power to impose restrictions on any and all forms of commerce, even those which occur entirely within the confines of a single state, what is the purpose of the words [quoting from memory] "...among the several States, and with foreign nations, and with Indian Tribes?"
It is by no means necessary to find that all of the words in a law change the meaning compared with what it would be in their absense, but they should at minimum allow a reader to think some particular interpretation is more or less plausible than it would be in those words' absense.
For example, the opening phrase of the Second Amendment: "A well regulated militia being necessary to the security of a free state" is not restrictive the second part, but does clarify that the "arms" referred to in therein aren't just "hunting and sporting firearms".
So returning to 1.8.3, what interpretation is either made possible or precluded by the "...among the several states, etc." language? Why did Madison not just write "to pass laws related to Commerce;" and be done with it?
Congress has no power to regulate the commerce which occurs entirely within the confines of a single state. Congress does have the power to regulate the commerce which occurs entirely within the confines of a single state if that commerce has a substantial effect on the interstate commerce that Congress is regulating.
"A well regulated militia being necessary to the security of a free state" is not restrictive the second part,"
I don't want to go off on a second amendment tangent here, but a number of lower federal courts have ruled that it is restrictive.
I'm assuming the above is your opinion?