“Due process” from my understanding typically meant going through a trail process. So, for example, someone commits a crime, they go to court, the jury convicts them, and then the go to jail/prison. Now, I could be a bit off, but after watching most of the lectures and reading the professors’ books, that’s what “due process” entailed.
To further that point, if “due process” meant what your second paragraph asks: “Wouldnt the law passed by Congress (presumably elected by the people of the states which outlawed slavery (in the territories) be considered due process of law?” The problem that arises with that is with Congress potentially passing an unconstitutional law. The proponents of the Consitution continually reassured the States that laws that are unconstitutional are not law at all. So, just because Congress passes a law doesn’t mean “due process.”
While I don’t know enough about the arguments being made about slavery in the territories regarding the 5th amendment to answer your question with confidence, I do know that one of the key arguments was that outlawing slavery was unconstitutional because it prevents slave owners from moving into these new territories that were, as you mentioned above, common property of the United States. Take, for example, the end of the Mexican American War. A political battle erupted after that regarding slavery in the newly acquired territory with the argument by Southern States being along the lines of this, “We fought and died and bled for that land as much as the Northerners, so why are we not allowed equal access to the land?” While I’m paraphrasing, I happen to agree with this argument and believe the Southerners had a very strong case constitutionally against outlawing slavery in newly acquired territory.
I’m sure the professor will clear anything up and/or correct anything I might be a bit off on.
Hope this helps,