Hunting indeed offered NCL-AMS-DUS in the mid 1950s with Vikings and Viscounts.
I am also aware of BKS serving DUS at some point between 1959 and 1963: NCL-LBA-RTM-DUS. Twice weekly in 1960, flight time a leisurely 4hrs. BKS main route form the Northeast to DUS was always LBA-(XXX)-DUS back then. It was served continously for more than a decade. In 1966, BKS also tried a connecting MME-AMS-DUS service that met the LBA at AMS, but that was short-lived.
I don't think BUA/BCal operated to Düsseldorf, but I may be wrong, of course.