The ubiquity of slavery in ancient history should be apparent in reading any histories of the era. It existed in all state level societies. As for levels of violence in pre-state societies, the bibliography of Pinker's "Better Angles of Our Nature: Why Violence Has Declined," has a copious list of sources. Both modern observation of hunter gatherer societies and forensic archeology converge on extremely high rates of homicide, significantly higher than Europe even if you take 1914-1945 as your measuring period, or the nations with the highest homicide rates today (concentrated in Central America).
I mention Africa in terms of neopatrimonial political systems because SSA has the best examples of pure neopatrimonial models, and Francis Fukayama draws most of his examples from the region in his "Origins of Political Order," and "Political Order and Political Decay."
(IMO, by these two volumes are by far and away the best works on state development. Not so much because of Fukayama's own insights, although those are good, but because he cogently summarizes the insights of Weber, Machiavelli, Jared Diamond, John Mearsheimer, Daron Acemoglu and James A. Robinson, etc. while also showing the flaws in their models and creating a new synthesis. He also avoids endlessly selecting on the dependant variable, unlike "Why Nations Fail," or the Tragedy of Great Power politics.")
Anyhow, if the implication is that Africa's problems stem from problems other than its politics, you are, of course, correct, but the negative relationship between explicit neopatrimonial relationships and governance quality extends outside the region. Countries with governments set up to ensure given levels of representation by given ethnic groups, rather than open elections (e.g. Lebanon's history) are far more likely to enter a civil war and more likely to return to war if an ethnic based political system is used following a cease fire. This is a replicable finding, although obviously all IR studies deal with low N studies and lack of experimentation.
I don't know what you mean by "intentionally exploitative policies." This is obviously true, although you could argue that modern capitalism was less exploitive that the systems that preceded it. In any event, my point was specific to colonies.
It's certainly not a settled matter, but the balance of findings in historical macro economic analysis is definitely on the side of colonies being net money losers for European nations. They were pursued for prestige and strategic reasons, and had the side benefit of letting the well-connected loot the treasuries of European nations, but they were a net drain on the host nations, particularly later colonial projects during the 19th century.
This is true even if the incredibly extractive, downright genocidal Belgian Congo project.
Secondly, the nations that gained the most from colonies (analyses generally conclude Spain saw short term benefits from gold and silver inflows) were impoverished by the early 20th century and relied on "catch up growth," to grow near to the main European powers in terms of development. Even today, Spain and Portugal, with their vast, early empires are significantly poorer than France and the UK.
Meanwhile, Denmark's rise to being one of the most developed nations in Europe occured after it lost its colonies. Austria's development trajectory increased after losing its empire. Switzerland and the Nordic nations are the most developed in Europe, despite the lack of colonies. Finland and Korea were impoverished backwaters into the 1950s, and modernized via institutional reform, not colonization, meaning colonies are neither necessary, nor sufficient for development.
On the other hand, the Gulf States boast per capita GDP levels on par with Southern Europe due to natural resource wealth, yet remain authoritarian states with lower quality of life by most commonly used measures, due to lack of core institutions. In modern state development literature these institutions are normally represented as a three or four legged stool of: accountable government/some for of voting, rule of law, and a strong centralized state with a monopoly on violence. Sometimes a strong, independent, professional, merit based beaurocracy is a fourth leg, others it is rolled into the strong state definition. The risk of CRT reducing development to extraction is that it then follows that simply transferring wealth to marginalized groups will be enough to reduce violence and increase standards of living. Yet this experiment has been run, with many poor nations inheriting vast natural resource wealth, and the result has often been a small minority benefiting from said wealth.
As to immigration causing congestion effects for other immigrants, or immigration reaching a tipping point at which the host nation's populace experiences an increase in anti-immigrant and anti-welfare state sentiment, this is a finding that appears in the immigration literature over and over, and can be found on Google Scholar readily. For example:
https://www.elgaronline.com/view/edcoll/9781781001264/9781781001264.00013.xml
Aside from academic sources, you can also look to industry. 19th and early 20th century industrialists made no secret of their attempts to intentionally hire a diverse workforce because it reduced the risks of worker cooperation and unionization efforts. Moving to today, Amazon had a leak showing that it also pursues diversity as a means of reducing the risk of unionization efforts, using it as a key metric of risk in statistical models.
Fukayama among others, lays out the case for reduced immigration being a factor in the homogonization of America after the 1910s. Whether or not it was the main factor is, of course, nigh impossible to prove, since there is a complex relationship between immigration, support for the welfare state, and unionization. What is certain is that curtailing immigration necessarily reduced economic inequality by reducing the number of low asset, unskilled workers entering the country, while also having modest to large effects on wages, depending on how you try to measure said effects.
As to Whiteness existing outside the US in the early 20th century as a unifying concept, I submit as evidence that it wasn't the fact that Europe experienced huge waves of ethnic cleansing (Germans totally removed from large swathes of Eastern Europe they had inhabited for centuries, Armenians subject to genocide in Turkey, the Holocaust of European Jews, the genocided in the Balkans, etc.). Racial theorists of the time also posited different European groups as different races. White, as an overarching identity shows up first as a meaningful social force in the US, and has gained relevance in Europe following the Post-War integration of Europe and the introduction of large non-European populations into Europe. Certainly a form of white identity existed in Europe prior to the 20th century, but it was not the inclusive identity it became in America.
Finally, as to: "The only way equality can be measured is by comparing outcomes," sure. The next step though, advocating for the elimination of anything that shows disparities in outcomes, is necissarily making the error of confusing correlation with causation.
CRT advocates have a real problem with doing absolute junk science, or badly misrepresenting the results of academic research, and then, when confronted with this, deflecting in an almost Freudian way with: "academia itself is a racist institution, and your disagreement is a sign of internalized racism/white fragility."
For example, the slide below is the definition of statistical error:
There are ways of measuring type I and II error in tests. There are ways of assessing their predictive power. The SAT, ACT, and GRE are not perfect predictors for academic and career success, but they are better than students grades.
Standardized tests for civil service positions were implemented with the exact goal of reducing bias in hiring. If the goal is to increase minority hires/admissions, then the solution might be to give those groups even larger preference on exams. The push to remove testing entirely isn't required to shake up admissions rates. A quota system would be the most effective means of doing that. I'd argue that tests are being jettisoned more because disparities in the test scores of those admitted/hired allow critics of affirmative action to use an easy quantitative means to critique said practices, rather than for any practical selection reason. Standardized tests allow bright students with poor grades due to poor quality, non-challenging academic settings to demonstrate their talent, which could be a boon for minority students.