That simply is NOT how genetics worked. If skin tone were one gene, everyone would be pitch black or Swesdish white, and would be dominant or recessive. The fact that there are so many ranges of skin color means that it has to be a product of many genes.
That is probably the next area of investigation. At least now they think they have the address of the genetic neighborhood to search for answers in. Explaining the variations in skin color that you note (and the other differences in physiognomy associated with them) will be difficult.
Exactly. Besides melanin content, we have nose shape, hair (kinky vs straight), eye color, susceptibility to malaria, bone density (Africans have denser bones, which helps with running, but impedes ability to swim), etc, etc