The Federal issue at stake was Article IV section 2. Your argument that "Illinois was a free state regardless of the Missouri Compromise " is tantamount to putting conditions on Article IV section 2 which are not in the text of the law.
The agreement of the founders and the signatory states was to clearly allow slavery consistent with the laws of the member states which had slavery laws. Changing the rules so that they can no longer exercise the same rights as they had when the agreement was signed, represents a breach of contract.
I don’t see how Art IV, Sec 2 has any bearing here. Illinois was a free state before the Missouri Compromise (MC), so Illinois was not affected by the MC. And the State of Missouri decided that Dred Scott was a free man.
Don’t see any relevant application of Art IV, Sec 2, Cl 1 (P&I), Cl 2, or Cl 3 (escaped slave).
Which is exactly what DiogenesLamp argues for -- a change in Founders' Original Intent based on Roger Taney's 70-year later reinterpretation.
Before Dred Scot no slave-holder and no slave-state considered it a "right" to take slaves into free-states permanently without freeing them.
So DiogenesLamp's arguments otherwise are pure sophistry.