To help your first point, Richard Dawkins clarified in The Selfish Gene that when we say "you have a gene for that," we're not talking about a specific gene. Obviously we can't point to the one gene that does X because genes don't work that way. We know that genes are responsible for all attributes, preferences, and behaviours and so invariably there is a gene or a series of genes that create a preference for thinness. To claim this is wrong because you can't name the exact gene is a fallacy.  Ultimately, you can debunk the entire argument with the fact that anything that exists does so because it was naturally selected. There is really no way around that. But she is clearly an "above the neck creationist" because, when you claim that evolutionary psychology doesn't exist, you're saying that the brain is magically immune to evolution and that everything that it is appeared one day. Obviously preferences evolved the same way anything else evolves.