You wrote:
“The US had zero reason to believe in 1939 that ANY Jews on board were in danger.”
Kristallnacht ring a bell? Yes, less than 100 Jews were killed, but everyone knew it hah happened - IN 1938!!!!
Sorry. Everyone knew in 1939 that Jews were in danger in Germany. The Jews on the St. Louis didn't go back to Germany. They went to Britain, France, Belgium and the Netherlands, where Jews were not in any kind of obvious danger. In 1939, France had the largest Army in Europe. It wasn't obvious to anybody in 1939 that France would be conquered by Germany in a month, nor that Germany would seek to exterminate the Jews when in 1939 it was simply trying to expel them.
The Nazis in the 1930s had the goal of expelling every last Jew from Germany. They worked to do so by making life very difficult for Jews (Kristalnacht being an example of this) and 90% of German Jews -- every last German Jew who wanted to leave (a few, mainly the elderly, did not) -- did leave.
In January 1942, the Nazis stopped trying to expel the Jews from their territory and embarked on a radically new approach -- extermination. That happened 3 years AFTER the St. Louis incident.