When and where was the name Kal-El first used?
I know Jor-El (formerly Jor-L) was in the earlier prints. But, when was the name "Kal-El" first used/created to designate Superman?
It's already in the earliest newspaper comics from 1939, though back then it's "Kal-L":
This is the extended origin of Superman story which was told only in the daily newspaper comics in 1939 (about a year after Action Comics #1)! This tale has never appeared in a comic magazine!
"Kal-L" isn't mentioned in Action Comics #1, but I don't know if that name appears in any of the later issues.
WRT the name-change from "L" to "El", Wikipedia says:
The earliest newspaper strips name the planet "Krypton", the baby "Kal-L", and his biological parents "Jor-L" and "Lora"; their names become "Jor-el", and "Lara" in a 1942 spinoff novel by George Lowther.
The entry doesn't provide a link to back this up, instead it refers to a 2014 book which points to a 1942 book as the source of the name-change:
Lowther, George (1942). The Adventures of Superman. Per Ricca (2014): "The book is also the first time that Superman's parents are named "Jor-el" and "Lara" — a slight spelling change that would stick."
The "per Ricca" refers to this:
Ricca, Brad (2014). Super Boys: The Amazing Adventures of Jerry Siegel and Joe Shuster – the Creators of Superman. St. Martin's Griffin. ISBN 978-1250049681.