You’ve got it. The CW was not about the continuation of slavery where it was established, it was about the expansion of slavery into territories and states where it didn’t exist in the particular southern form.
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.