You posed very interesting questions. I am not a neurologist, neuroscientist, or ED clinician. I am a clinical researcher with >15yr experience (in the process of a loooong interrupted PhD) who has perforce given herself a crash course in EDs. My undergrad background was Anth/Psych with heavy focus on the biological bases of human behavior, and I had intended back then to become an adolescent psychologist (who knew! the irony, it burns!). With that in mind; here is my shot at answering very complicated - and GREAT! questions.
1) I would say that any time you are running consistent macro nutrient (ie carbs, fats, protiens) or micro-nutrient (specific vitamins and minerals) deficiencies for extended periods of time, you run the risk of organ malfunction - including the brain. Your brain is "calorically expensive"; it takes a lot of carbs, evenly spread through the day to work properly, and lots of fats to maintain it's physical integrity. Additionally, the brain is a very complex electro-chemical processor. It needs appropriate balances of vitamins minerals to run properly. So any kind of malnutrition is bound to begin to affect the brain's functions. HOWEVER, malnutrition may or may not result in weight loss! Someone who is w/r but getting insufficient protein or fats will be adversely affecting many organs including the brain. Also, there is huge variability across people in how much malnutrition will have how severe effects. This is one of the reasons why BMI is such a poor indicator. The last complicating factor I'll mention is that, as with many diseases, if you've suffered from malnutrition once, you're more prone to it's effects if it recurs, so that someone whose had AN but is w/r will often start having AN symptoms with even very minor recurrence of incidental restriction like from a stomach flu or a busy day where a meal or two gets skipped.
2) The answers in the literature to this are Yes, No, Sometimes, and We Don't Know! Emerging studies seem to indicate that people who primarily develop AN versus BN are different from each other in ways that can be measured. However, this picture is muddied somewhat by the fact that it is known that about 50% of AN patients will develop BN symptoms. Another confounder is that some of the deficits and co-morbidities are the same between the two groups, although there are clearly tendencies that show up more often with one diagnosis or the other (eg, BN associated more with risk taking behaviors, AN associated more with perfectionist behaviors). IMHO the biggest caveat; Diagnosing physical pathology based on human behavior is rotten business, because human behavior is really complex. Our genetic heritage is our genotype. How genetics are outwardly expressed by an individual is called the phenotype. If you know someone's blood type - their blood phenotype, you can make some educated predictions about their genotype. No matter what has happened to a person, it won't change their blood type. But human behavior is HUGELY influenced by our experiences so trying to go from a person's phenotype to their genotype is really NOT something we can do at this point, and my best guess is that no matter how good we get a the genetics bit, you'll never be able to perfectly predict the genes from the behavior, OR the behavior from the genes.
3). Simple; MOST doctors are not taught about nutrition. I can say this with some inside authority - for 10yrs 1/2 of my job was coordinating the education of 2nd year medical students at a major teaching hospital. The entire nutrition component of four years of medical school for our students was about 3hr of instruction that focused mostly on adult hospital inpatients. And our curriculum, believe it or not, was one of the MOST comprehensive around. Unless the Dr. in question has sought out an elective rotation with more nutritional focus (at this hospital, that might not be until after med school, if at all), they have not received any comprehensive or applied education on the effects of nutrition on the body, beyond what is taught in basic physiology classes. Pediatricians certainly get nutritional information, but to my knowledge, it is more about physical effects on growth. We are only just starting to integrate behavior with underlying physical pathology in the brain, and therefore making the leap from nutrition = physical brain status, physical brain status = observed behavior is NOT something most doctors in the US have been trained to do.
Around the Dinner Table Forum - Hall of Fame > Understanding Eating Disorders > The Biology of Malnutrition >