Tony, I agree with your information about the variations on MOMME-SEN.The origin is probably Danish.I have been told and have read that the custom of adding -sen or -son is of Scandinavian origin, circa 1828.(The Danish Genealogical Helper, Vol. 1, Nr2, October 1968, p.,59)
HOWEVER, there is that area of Schleswig Germany that has gone back and forth between Denmark and Germany ... So although the spelling "MOMMSEN" makes most people think my heritage is Danish, we claim to be German from Schleswig, Holstein area.