Questions about Tolkien's legendarium

Short answers, pulled from the story.

When did J. R. R. Tolkien begin writing poems and story sketches for his mythology?

J. R. R. Tolkien began writing poems and story sketches in 1914 as a private project to create a mythology for England.

What was the first complete version of The Silmarillion written by J. R. R. Tolkien?

The first complete version of The Silmarillion was the Sketch of the Mythology written in 1926.

Who edited and published The Silmarillion after J. R. R. Tolkien died in 1973?

Christopher Tolkien chose portions of his father's unpublished material and shaped them into The Silmarillion in 1977.

Why did George Allen & Unwin reject the Quenta Silmarillion submitted by J. R. R. Tolkien in 1937?

George Allen & Unwin rejected the work because they found it obscure and too Celtic.

How many letters from 1951 to 1955 did J. R. R. Tolkien use to describe his works as a legendarium?

J. R. R. Tolkien described his works as a legendarium in four letters from 1951 to 1955.