For Russell the ransom was the main theme of the Bible. For Rutherford the main theme of the Bible was the vindication of Jehovah's name, and the ransom was secondary.
JWs can change any belief or doctrine they wish. The could claim God produced an Adam and Eve by evolution, and sin came through them, which I think is a position some Christians take. Or they could redefine their understanding of the origin of sin.
Or they could cling on to their version of creationism indefinitely...