Unfortunately, abolitionism can never gain any traction until slavery becomes economically unviable

. That would require dramatically sped-up technological and economic development in Europe, which may not be possible without American resources. Even if the European kingdoms had all banned slavery (and as IIRC it had almost disappeared there that might be conceivable), I doubt such laws would be enforced outside Europe. You'd have to either posit stronger, stable states in West Africa or that the extractive plantation colonial system never be established.
Of course, if West Africa has strong, stable states, the slave traders would probably just get their slaves from somewhere else, probably from East Africa through the Arabs. Native American mass slavery is not an option due to disease.