I am not sure why you think the audience had tribal hostilities against him. These people were not klansmen, they were young British aristocrats whose parents were overseeing the deconstruction of an empire on the basis of the enlightened principles of the day. Baldwin was not criticizing them particularly, he was criticizing the crude upstarts from across the pond. I'm not aware of any good reason to believe they were not already rooting for Baldwin before the debate took place. But either way, these people knew very well that life was not nice for people that had been recently conquered or enslaved. He did not need to lure them into seeing that.

I believe the applause was entirely on the basis of the oratorical trouncing he gave to Buckley, who was the wrong person to be on that stage, and as a result of the fact that they were already sympathetic to his point of view. What impact would Baldwin's presentation have had on a group of British aristocrats 200 years earlier, who were making fortunes by financing slave ship voyages?

Would they have been convinced that the cruel behavior of some sheriff toward a black woman was evidence that treating Africans differently from Europeans was poisonous to their minds?