The problem with this diet is that it wouldn't be a diet; it would be many different diets. Lots of people are lactose intolerant and it would be stupid to remove dairy products from the diet of those who are not. Likewise, a vegetarian diet is not a "variation" of a non-vegetarian diet.

Also, why are you talking about 1900?

Maybe, indeed, human ethics are so dependent on alleles that vary within the population and chance environmental factors that CEV is impossible. But there's no solid evidence to require assuming that a priori, either.

I think the fact that humans can't agree on even the most basic issues is pretty solid evidence. Also, even if everyone had the same subjective ethics, this still would result in objective contradictions. I'm not aware of any evidence that this problem is solvable at all.