OK, so what about the scores of relatives in Kenya (and elsewhere) who claim Barack Obama, jr as kin via Barack sr, and Stanley Ann? This would clearly make Barack, jr, the son of Barack, sr, in contradiction to your assumption that an unknown child was taken into adoptive custody by Stanley Ann and parents.
They are all gaining something from doing so. Mark says straight up on the record that he got $200k up front with another $400k to come if he keeps his mouth shut until Zero is out of the White House.
And, not to say that Zero is not in fact born somewhere in the clan although at present, I tend to doubt it.