Thank you for that wonderful post, Alan. I've always felt that Nathan is clearly supposed to be a Jewish character, even if I knew only a small fraction of the supporting evidence you gave. I also wondered if maybe some of the characters in Nathan Detroit's world might be Italian, though I think it's pretty clear from his name that Big Jule is supposed to be Jewish :-)
On that note, I find it interesting that Adam Gropnik chose the Sinatra, an Italian, as his example of someone who tended to "speak high" when he spoke for public consumption, though I'm not sure I can offhand think of a famous Jewish performer who might also fit the category. Maybe Alan King?