When and where was the name "Kal-El" first used?

It's already in the earliest newspaper comics from 1939, though back then it's "Kal-L":

According to this site:

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.

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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.

The Wikipedia entry on the 1942 novel The Adventures of Superman says that

It was Lowther who first provided many now-familiar details of Superman's birth and early life. The first two chapters provided the first detailed description of the planet Krypton, and renamed Kal-El's parents Jor-El and Lara who previously had been named Jor-L and Lora.

Note that the book actually spells it "Jor-el" (as you can see in the scan below), it would later change to "Jor-El" in the comic books.