The noah story is an ancient myth shared by many different cultures. Reading into it as a story of historical truth always appeared rather silly to me. We end up imposing our own sense of morality and justice on a tale that was told to influence and entertain people that died long ago.
Of course, when a person accepts that the story is sipmly a myth other long held assumptions are challenged (e.g. the Bible is historicaly true cover to cover).
One suggestion you may want to think about. Do you really care about the moral implications of the Noah story? It seems as though you are trying to rationalize your arguemnt for why you believe the story to be myth, without coming out an saying it. I would personally stay away from argumentation like this because it appears to make you look overly picky. I would compare your argument to another one I have heard some ex-jws use from time to time, this issue of "who will be resurrected from sodom and gomorrah". Sure the WTS has changed their position on this teaching, but does it really effect your life in a significant way? Many other Christian groups believe the Noah story is true as well, but that doesn't make them shun their family or refuse life saving medical treatment!
If I was you I would be careful not to come across as antagonistic, especially for the sake of your wife. I went through a simliar situation and can tell you that the less conversations you have with Elders the better!