Does the Hebrew word in the plural number translated God in Genesis one, prove the author was a polytheist?
No. In the Hebrew tongue words often have the plural form without the idea of plurality. The language was not exact in this particular, like modern languages. The English reader can see this in the first chapter of Genesis; for, although God (Elohim) says in one verse, “Let us make man in our image,” in another he says, “I have given you every herb bearing seed,” etc.; thus using at one time the plural pronoun and at another the singular. The later could not have been used had the Elohim meant a plurality of gods.