Who coined the name "Earth"?
As the writer of the article I've pasted a link to says, it's impossible to know who named the Earth. Fundamentalists of any religion will say it was God. But we're going to have to let the linguists tell us how we got the word. The same article states that
Earth is Old English and German in origin, related to the Old Saxon 'ertha', the Dutch 'aerde', and the German 'erda'. Terra is a French and Latin word, and so isn't part of the 'Earth' etymology.
Wordorigins.org recognizes the Old English and Germanic uses of the word but notes that the root word goes back to the Greeks. It also adds:
Earth was not used in the sense of a planet, like the others, until c.1400. The adjective is Terran, 1953, and is most commonly found in science fiction. The adjective is after Terra, the Latin name for earth.
The Compact Oxford English Dictionary also gives the word "earth" an Old English origin.