We're talking 2-6 megayears, depending on how you define the earliest ancestor of humans. This long span of time means that it can simultaneously be true that, as you say, most people stay home; while at the same time some small percentage went on walkabout. The walkers were sufficient to spread individual instances of the human genome to remote locations, where it could diverge a little bit; and then when they returned home they reintroduced those different, and presumably somewhat improved, genes. The tens of thousands of years involved in these circulation cycles is a blink of the eye in evolutionary terms.

I find the notion of human evolution powered by a "circulating genetic engine" to be a more compelling explanation for the relative uniformity of the human genome--or as you would have it, in your eugenecist style, "genetic mixing"--than the cliche genealogical chart. A genealogy implies unidirectional movement and permanent separation of subspecies and smaller units of population (what we call "races"), and that doesn't match what we see today. You emphasize the differences in various populations, I see the similarities.

I don't agree with the concept embodied in "small genetic contribution...smothered by the genes of a native population". Genetics doesn't work that way. If a small genetic contribution is an asset to the species, selection will favor it and spread it. A new genetic combination will only be "smothered" if it's maladaptive. Genes aren't an amorphous soup, in which only quantity of "particles" matters; genes represent a highly structured information system that persists over long periods of time.

To take the longer view, I think that this focus on the human genome and its concrete manifestation in human bodies is a thoroughly passe view of humanity. The defining characteristic of our species isn't the shape of our bodies or the color of our skin, or even the size of our brains. It's the extended phenotype we create in the form of human society...and it's human society that is evolving, not the human body. Social evolution is many orders of magnitude faster than biological evolution, and it's the sole reason that we're able to cope with sudden catastrophe and rapid change. We organize our way around problems, mobilize entire populations, direct intellectual and material resources at various problems...society responds to the big challenges, not individuals.