Dude. The NWO was unconstitutional. Look up Article IV.
You might just also search through the old ACW threads for the place where you, Non-Sequitur, 4CJ, rustbucket, I think Pea Ridge, and one or two other people had a blazing gun-battle over the constitutionality of the NWO. I stayed out of it pretty much, iirc, because it was, well, "your issue", and I was pretty much a spectator. It was all entailed in a larger discussion of Dred Scott and the "personal liberty" laws of the Old Northwest States. (Illinois did have slavery for some time after statehood, since the French had introduced it in the 18th century iirc.) There was a big discussion about Illinois and their history of emancipation, and another about Pennsylvania and their partial, limited abolition which converted chattel slaves to indentured, and specified a term of indenture for slaves thus converted, in their bondage/modified abolition act.
I'll let you guys do the looking who are better at cruising old FR threads than I am (I'm on dialup and have not-so-hot search skillz).
Well you better let guys like James Madison and a dozen other signers of the Constitution know of your opinion since they voted and approved an identical bill to the 1787 Ordinance during the very 1st Congress under the Constitution in 1789.
I don't know what you see in Article IV. I see it gave Congress exclusive control over the Territories including the power to prohibit slavery if they so desired.