It's a 'through gut' D, so it's one-way traffic. Stuff goes into it, and keeps going into it until the octopus cannot eat anything else. From there it will be drip-fed into the stomach, then spiral caecum, and eventually (with some storage of something in the digestive gland via 2 ducts .... not quite sure what goes on) enters the intestine and is expelled via the (cough). The crop is, as you say, definitely just an enlargement; some species have a diverticulum of it - probably just serving to increase storage area for these gluttons - but some do not. They used to believe that the presence or absence of this crop diverticulum had some phylogenetic value, and it was used to differentiate the genus Bathypolypus from Benthoctopus, but we now realise this to be an oversimplification, and its phylogenetic value to be far more limited (this was a large part of my MSc back in the early 90s). That's not to say that it has no systematic value - a species may lack a crop diverticulum but another may have it, but not all species that lack a diverticulum of the crop are more closely related to each other than they are to species that have one. The same applies to species with and without an ink sac - you cannot lump them together into subfamilies, as the ink sac has been lost many times over in the evolution of benthic octopodids.