Nearly all the Mediterranean cultures have a myth eerily similar to Noah's flood. Mesopotamian myth mentions a great world-ending flood as well.
The story of the different versions of humanity that the Greeks believed came before us can be found here. https://en.wikipedia.org/wiki/Ages_of_Man
Absolutely.
Another problem has always been DC themselves. As has already been mentioned, they've simply come up with every excuse under the sun to not do more with her. Finally? They took a chance, put Diana in an excellent movie.......and lo and behold! Big ol' piles of money and a ton of positive press!
If this doesn't motivate them to see that Diana can make them at least nearly as much money as Batman can, I don't know what will.