Africa is home to the tallest (Sudanese) and shortest (pygmy) humans. You can think of the entire non-African population as a little inbred offshoot of the main stock.

I recommend E. O. Wilson's "Social Conquest of Earth" as a reference for quantifying the genetic difference within and between populations.

I wouldn't necessarily call them inbred. Whites crossbred with Neanderthals and Asians did the same with some other related Homo genus whose name escapes me.

Doesn't surprise me that both went extinct. If we're still this prejudiced towards each other in the 21st century, I have little doubt that we played a large role in their extinction

Denisovans (Homo Altai / Homo sapiens Denisova).

You make a good point that the emigrants did mix in a peppering of genes from their early-arriving distant cousins. But overall, only a tiny fraction of the available gene pool made it out of Africa.