Taney’s finding that Congressional bans on slavery within territories were unconstitutional was, I believe, complete nonsense. The Constitution specifically empowers Congress to make “all needful Rules and Regulations” regarding territories. Congress had been enacting territorial slavery bans without controversy since 1789. Many framers of the constitution voted for and signed such bans.
Likewise, the finding that no black person, enslaved or free, could ever sue or be sued in federal court under diversity jurisdiction was ridiculous. Diversity extends to controversies “between citizens of different states”. Some states such as Massachusetts had explicitly recognized black people as citizens; Taney had no power to override this. He fantasized an argument whereby a citizen of a particular state might still not be a citizen of the United States, which was both irrelevant and wrong.
Taney would have been a surer ground if he had pointed out that Missouri was not one of the states which had recognized black people as citizens, so Scott, even if free, was not a citizen of Missouri. Missouri subjected free black people to extreme disabilities which were probably incompatible with citizenship. But on the other hand, Missouri did allow free black people access to its courts. There had been hundreds of freedom suits in Missouri. Taney could have argued that Missouri was free to allow non-citizens access to its state courts, but this carried no weight in federal court because federal jurisdiction was explicitly limited to citizens. He could have made that argument, but he didn’t. The argument that he did make was nonsense.
Finally, did Scott’s residence in Illinois and Minnesota Territory emancipate him after he returned to Missouri? Taney said no, and on this limited question I believe he was right. Or at least, legally defensible. For Scott to be free, Missouri would have to apply the laws of Illinois and/or Minnesota which said that he was free. This is a matter of “interstate comity”. My limited non-lawyer understanding is that state laws have extraterritorial force only by the consent (comity) of the forum state. Missouri had granted such comity, with respect to freeing slaves based on Northern state laws, until the 1840’s.
Then, they stopped. Scott’s case began with a state suit against his owner Emerson, who was a Missourian. The Missouri state court overrode its own precedents and refused to apply comity, remanding Scott to slavery under Missouri law. Then, Scott initiated a federal case against his new New York-based owner, Sanford. Taney was justified in applying the most recent and most apt Missouri precedent, Scott v. Emerson, to the federal case of Scott v. Sandford.