I don't think this is a case of "which evidence would change who's position" but more of "let's hash out what we disagree about exactly".

Maybe a better phrasing of my position would be: I don't think that American-style institutional slavery requires irrational thinking (except possibly about morality, but I don't know hot to reason clearly about morality especially in different social contexts). However, it's quite likely that in a political context where large numbers of people with no particular stake in slavery (in this case, northern voters) have the power to end it, maintaining slavery may be easier by spreading (and believing) false beliefs about slavery and the lives of the slaves. So in that second meaning, I would agree that slavery requires false beliefs.

In any dispute that is to be settled by public opinion (which covers a lot in a democracy), both parties can be expected to resort to dark arts. That doesn't mean that either side holds wrong beliefs - maybe both do (protestants vs. catholics in the Wars of Religion), maybe neither does and it's just a conflict of interests (The old nobility vs. the new bourgeoisie in the French Revolution) ... but in all cases the side that stops resorting to dark arts and stops being fanatically devoted to it's cause is more likely to lose, so in a sense it "requires false beliefs".