I don't understand your point. If the Constitution requires a state to return escaped slaves, how are you going to create a "free" state?
I suppose you could pass a law that won't allow anyone born in that state to be a slave, but I don't see how you can do anything about people moving into another state and bringing slaves born elsewhere with them.
The point is that a lot of CW arguing ignores the dilemma you describe. Slave-holders saw themselves as abiding by the Constitution (and the Declaration), and Abolitionists, driven by a novel interpretation of Christianity, sought to change the historic status quo.