And this is a valid and objective point from you. I have seen others argue that the Slave holders expected the same sort of "Executive Order" bullshit and Executive Departmental foot dragging and obstinacy, interfering with their rights in the manner Obama uses today to accomplish most of his agenda.
There were refusals to enforce the fugitive slave laws, and every legal or sophist trick was being used against them; the events that triggered the Dred Scott case being an example.
Yeah they were lying, and were doing anything in their power to refuse to comply with the deal they made back in 1789, or the actual and correct meanings of the law. Much like with many issues of today, such as "Gay Marriage".
One entirely constitutional way of putting a major crimp in slavery would have been for Congress to prohibit interstate trade in slaves. I’ve never seen this discussed anywhere, however, neither then or now.
BTW, Lincoln agreed fugitive slaves must be returned. He just insisted on reasonable due process to determine they actually were slaves, which seems reasonable.