It was specifically the rights of one state to force another state to enforce slavery

Exactly.

And the Northern states and the Feds were cool with maintaining the status quo of legal slavery until halfway thru the Civil War.

So if the South hadn't gotten greedy and tried to force a strong federal government, slavery would have stayed legal. But they tried and both won and lost at the same time.

They got the strong federal government they asked for, it just wasn't on their side.