Never heard of pythonscad before, but I've always thought it would be a good idea (if only I honestly had the skill to make this myself). Thanks for the links!
I think a mixture is probably the best, because, cynically, I bet people with high social capital could engineer a way to improve their standardized grades as well. By requiring them to cover more bases, it drives their costs up. A ringer is an obvious solution, but there is a lot of room to be "creative." The best way for admissions to pick well is to always remain liquid on metrics. Keep the pool wide; diversify your metrics portfolio.
The 1800s were brutal everywhere, but slavery is an inherently corrupt system. I can certainly imagine merciful white masters who genuinely cared for their slaves, but slavery is such a cut and dry obvious violation of human rights that it's not worth considering. A single person being considered, even in the most benign technical sense, property is an atrocity. We can and perhaps should commemorate those who worked very hard to salvage justice in an inherently just system, but we're still trying to wash the stains of racial injustice within our society today. Any time spent uplifting these persons must unfortunately be set aside to provide equity to those long ignored.
All of that being said, I can't say for certain that you're being intellectually honest, you're conflating quite a few things with crucial nuance. As an example, you claim black slave families were closer and more stable, because they were penned in and forced to breed together to provide additional laborers who would replace them. Have you ever considered that divorced families are better off than unhappy marriages? Have you considered that perhaps marriages aren't entirely necessary, and modern black culture isn't well suited to traditional marriage norms codified by white JudeoChristian people? If you're heart is in the right place, I'd imagine you could tell us a story of a true ally to black slaves in the south. One whom was raised in a toxic racist environment, but learned to work within a inherently broken system to provide some sense of equity.