"If economic superiority depended on cheap labour how did the Northern non slave state do so well compared to the southern states? And why did the USA maintain its dominance since WW2 despite average wages in the USA far higher than developed European and Asian nations?"

The Northern states did well because of slavery as well. Not only were there slaves of their own but Wall Street financed slavery. Why we continued to do well after the Civil War and WW2 is because we replicated slavery with first the Black Codes and then Jim Crow. We continued to allow slavery through a loophole in the 13th Amendment which encouraged mass incarceration because prisoners could still be slaves.