Ron should be able to return The Eternal to Diamond if it has been mis-solicited. I have done this before. I returned an art book that was not solicited as adult, but the opening page showed two women having sexual relations. It may depend on how good your relationship is with your Diamond rep.
I have voiced my concern to Diamond several times over the past few years (to no avail) about the self-rating that is included in Previews. Diamonds' answer is that the company that solicits the product decides whether it is mature or adult. Can you see a problem with that plan? Not only are 'mature' and 'adult' not defined for the retailer, different people will rate product differently based on a variety of things. Different parts of the country have different moral standards. Maybe the manufacturer thinks that a certain rating will garner more (or decrease) sales, which would make their decision financial related. My understanding of 'mature' or 'adult' may not be the same as others. I have ceased to order ANY Avatar Comics for my shelves because of their insistence on placing 'adult' ads in the back of the comic no matter whether the comic is 'adult' or not. It is like the movie theaters who show R-rated trailers at PG-13 movies (or on video tapes).
I know no one in our day liked the comics code, but at least it was supposed to be consistent. There is no consistency in the current system. It is my opinion that nothing deemed 'adult' should appear in the Previews catalog. There is a Previews Adult for that purpose. I sell Previews catalogs to anyone who wants one, but would prefer they not offer my customer Playboy, or adult comics and books. I, as a retailer, should be able to choose whether to carry adult related items (or 'adult' related items disguised as 'mature'). I don't like the confrontation, which occasionally occurs when I tell a customer I won't order adult items he sees in the Previews.