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But state responsibilities for capturing and returning Fugitive Slaves were eliminated by the Compromise of 1850 Law, which transferred the job from states to Federal Government.

The Constitution is not amended by "compromise". It is amended by a clearly enumerated process. Nothing short of a constitutional amendment will repeal that clause in Article IV.

Till that is done, states are obligated to do what the constitution says, or am I mistaken about this?

You see, if you argue that states don't have to do what the constitution says, I fear there goes most of your larger argument.

DiogenesLamp referring to the original Constitution's Fugitive Slave clause and Compromise of 1850 Law: "The Constitution is not amended by "compromise".
It is amended by a clearly enumerated process.
Nothing short of a constitutional amendment will repeal that clause in Article IV."

But no challenge was ever made in the Supreme Court on constitutional grounds, to the Compromise of 1850 Law, which moved responsibility for returning Fugitive Slaves from Northern States to the Federal Government.
Slave states which objected to the Compromise of 1850 could easily have challenged it in Court, and with Judge Roger Tanney its Chief Justice, likely have succeeded.

But of course, they did not, since it was a Compromise they heartily supported.