I don't know that I can add much to your already thorough arguments. Just a couple of small ideas:
Your point about the slender woman is related to another one, which is "look good on their current diet compared to what other diet"? Even if a person has a broader range of looks based on what they are eating, they probably don't go from looking horrific to looking spectacular. Someone who has a propensity to having acne, for example, may go from acute to mild symptoms based on diet, but never have the clear skin that another is blessed with no matter what he eats. We all have different weaknesses, and some are more in plain sight than others, which brings me to another point: Maybe they look relatively good, but they're depressed, or constipated, or have terrible blood glucose control.
SecondHealth is not always visible.
Also, the effects of a diet can take time to show up. For instance, now that I have the experience of having lost a lot of weight, when I see a fat person, I don't automatically necessarily think they have a bad diet. For all I know they have lost 50 lbs this year so far, and their diet is fantastically healthy.
Finally, there are people who are the opposite of your example. Some people have metabolic, genetic or other physiological problems that are beyond the scope of just dietary intervention and aren't looking very different no matter what they eat either.