Fantastic research work, Binarysunrise! As for what happened to the 1940 World's Fair "Superboy", there's the threads on this very message board:
http://superman.nu/phpBB2/viewtopic.php?t=2708http://superman.nu/phpBB2/viewtopic.php?t=2783And I find it absolutely incredible that a "Superman's Girlfriend" costume was marketed as early as 1955 -- three years prior to the Magic Totem Super-Girl (who wore the same red skirt), and fours years before Kara Zor-El's arrival. You wonder if DC wasn't watching sales of this costume when debating on whether to introduce a Supergirl.
This ad might make an interesting historical oddity in the "Supergirl Costumes" section:
http://superman.nu/supergirl/costumes/