Count calories . When food is consumed it has a certain caloric (heat, energy) content. They are really only three ways that that caloric content can leave the body. 1. Malabsorption (i.e. You don't consume it in the first place, e.g. Diarrhea). 2. Burned off as heat. 3. Burned off as movement. Anything left over is stored as fat. Overthinking diets and body type clouds main concern: reducing caloric intake.
Yes. Yes. The "ketogenic diet" up until recently was simply the diet everyone ate except for a small period of time each year just prior to winter when you would load up a bit on carbs and have a nice fat layer for winter. Metabolic ketosis is healthy (and natural) for you as long as you are a mammal. The easier it is for you to gain fat the more helpful it will be.
No. The best answer is to change your lifestyle. Learn to cook healthy food- find some great fruits and vegetables you love and make them the mainstay of your diet. Avoid salads with lots of dressing, and know that if you eat a lot of meat- like more than 4-6 ounces, it will simply become fat. Diets don't work- lifestyle changes do.
Lifestyle change. Your physician can give you better advice. "endomorph" is a subscientific term rather than a true body type, let alone a destiny. I suspect that even healthtap members who are sympathetic to various "pop" diets will mostly agree that in the long-term, you help will come from reprogramming yourself about fitness activities & healthy eating. If you are very obese, consider bariatric surgery early.