I probably shouldn’t allow myself too many caveats, since this thread is supposed to be about what we know with 100% certainty, but I should back up #2 with a couple.
Caveat 1) Two individuals of the same weight and height may not have the same basal metabolic rate (BMR), mainly because of differences in body fat percentage, bone and muscle mass, and age. The research I have looked at suggests that more than 70% of variation between individuals can be explained by those factors. However, the rest of the variation might be explained by thyroid status, exercise, diet, genetics, or other unexplained factors.
Caveat 2) Not everyone digests everything they eat, and certain foods are harder to extract calories from.
Nevertheless, allowing for these caveats, I don’t believe there’s much an individual is able to do, in the short-term, to change his or her BMR. As far as I can tell, building muscle mass would probably be the most obvious way to increase it, and losing muscle mass would be the easiest way to decrease it. But it takes a long time to build muscle (a pound a month is doing well), and you’re not going to lose much muscle unless you’re starving yourself or dieting too much.
So I believe that under normal conditions, weight gain and weight loss can be controlled by monitoring food intake and activity levels. Psychological factors can make this process enormously difficult, but I don’t think the chemistry is all that complicated.
I know you’ve complained about not being able to gain weight despite eating a lot. But you also exercise a lot, and (if I remember correctly), you eat a lot of high fiber foods, like oatmeal, that can hinder calorie absorption. Or maybe your family has a naturally high BMR because of genetics, in which case you’d just need to eat more calories to achieve balance.
I’ve also read that, while obese individuals tend to underestimate the amount they eat, and overestimate their exercise, people with eating disorders do the opposite. They think they are eating more than they actually are, and they think they are exercising less than they are. This could also be a factor in your situation.