(1) Tribalism, as the term is used in social psychology, is a general human capacity. There is no apparent cognitive difference between groups drawn along mutable or immutable features. You get the same preference for the in-group, the same skepticism of the out-group, the same harshening of moral judgments against the out-group, the same forgiveness of moral blame on the in-group, the same reaction delay when pairing the disfavored group with a term of positive affect, and so on. (The Implicit Association Test was developed by social psychologists, not social justice activists, and was shown to work on positive/negative word pairs as a control before being used on social matters.) It is in this sense that I say tribalism is necessary for racism: this cognitive ability of humans is deployed in the case of racism.

(2) The economic realities that made slavery profitable in the 1800s are long gone. That capitalistic interests are served rather than hindered by decreasing the productive output of a group of people, while draining resources from society to disproportionately give them government assistance, requires a detailed argument, not mere assertion. Perhaps you have written on this specifically, but if so, I haven't come across it yet. The primary alternative hypothesis is that it's just tribalism, now operating independently of the original justification.

(3) I am about to read more of Bell, so I will refrain from further comment, as it would be more productive to be better informed; a handful of quotes and select passages is perhaps not enough to render a sufficiently considered judgment.