Ditto here, unfortunately.
[rb]: States rights does not include the right of a state to violate the Constitution.
[SL]: Nope. But certain actions taken by the federeal government to enforce a constitutional provision could be a violation of states' rights. The clause you quote contains no prescribed enforcement mechanism other than presumably for State 1 to sue State 2 in federal court.
The Supreme Court ruled the Fugitive Slave Law of 1850 constitutional, and Lincoln said he would enforce it.
Another response to Northern laws that frustrated the Fugitive Slave Law was secession. Massachusetts' Daniel Webster said to Northern states concerning fugitive slaves, "A bargain cannot be broken on one side and still bind the other side." I say: "You don't play by the rules you agreed to when the game began, then don't be surprised if the other player leaves." (Webster's quote is more eloquent than mine.)
There were concerns about the constitutionality of the Fugitive Slave Law. It didn't provide for trial by jury and the presumed slave might not have been able to question the witnesses against him. However, slaves were property under the laws of many states and didn't have a standing in those courts.
There were also concerns about the constitutionality of some personal liberty laws used to frustrate the return of fugitive slaves. From the Philadelphia Public Ledger of December 20, 1860:
Chief Justice Shaw, B. R. Curtis, Joel Parker, and other citizens of Massachusetts equally distinguished, have addressed a letter to the people of that State on the Personal Liberty Bills, which they declare to be unconstitutional. They urge strongly the repeal of them ... conspicuous and palpable breaches of the national compact by ourselves ...
Lemuel Shaw was Chief Justice of the Massachusetts Supreme Court. Benjamin R. Curtis was a former Associate Justice of the US Supreme Court. He resigned from the Supreme Court in protest of its Dred Scott decision. Joel Parker was professor of constitutional law at Harvard and former Chief Justice of the New Hampshire Supreme Court.
The Massachusetts law made it more expensive than the slave was worth to try to get him returned. The last fugitive slave returned from Massachusetts was in 1854, and there were large numbers of fugitive slaves living openly in Massachusetts.
It's worse than that. The Dred Scott decision claimed that all persons of African descent (presumably meaning any trace of African blood) were by definition not citizens of the US and had no standing in federal courts. Given the direction things were trending at the time of the KN Act and the Scott decision, it is quite possible the Court would eventually rule that "Africans" could have no standing in any state court, and that a US citizen could not be prohibited from taking his property into any state, thus making slavery legal nationwide.
The Scott decision was factually incorrect in many areas, and stated that Africans were citizens of no state at the time of the formation of the Union. This required ignoring the fact that free men of color were full citizens under the law in several states at the time. Ironically, North Carolina was one of them. This inconvenient provision was gotten rid of by a new state constitution in the early 1800s.
Whether blacks were allowed to exercise their rights as citizens in these various states is of course another question entirely.