" A person weighing 300lbs has roughly the same neurochemical hunger/stress response as a person weighing 150lbs,"
Not sure about this. Will depend on the type of (over) eater. Do they over eat because of psychology (stress reaction), do they over-eat due to some hormonal imbalance (lower gut hormone despite eating) and subject keeps eating. Then there's binge eaters. Each of these three types of over-eating have a different "neurochemical hunger/stress response" to a normal person who has eaten enough and stops. [0]
You're missing the point: a 300lbs person who stays 300lbs is not over-eating. Over-eating isn't biologically defined against some ideal meal size, it's defined against homeostasis: consuming as many calories N as the body burns. That's what the brain's hunger feedback mechanisms are supposed to care about.
Carrying around extra fat causes N to go up.
"Getting fat people to go on diets that cause them to lose weight" is approximately as difficult, motivation-wise, as "Getting skinny people to go on diets that cause them to lose weight". It's extraordinarily difficult, because their bodies are screaming at them to go back to the refrigerator.
The novel question to answer is why overeating - consuming more calories than you burn, and thus gaining weight steadily over long time periods - is so much more prevalent now than it was in previous decades. Why people who are already fat are inclined to eat more (much more) than skinny people, is the simply metabolic reality of the body's drive for homeostasis.
There is a healthcare problem, of course, with having fat people around, but the scientific question of what is making them all have weight problems at higher rates than before, is somewhat distinct from the practical problem of how we get the existing fat demographic to lose weight. Aggressive funding of bariatric surgery seems to be the least-bad method of the latter for the moment; Almost nothing else can be shown to work effectively at the levels required, since we're effectively fighting the strongest drive the body has after breathing.
These are good points. This one I'll have a stab at.
"The novel question to answer is why overeating - consuming more calories than you burn, and thus gaining weight steadily over long time periods - is so much more prevalent now than it was in previous decades."
Not sure about this. Will depend on the type of (over) eater. Do they over eat because of psychology (stress reaction), do they over-eat due to some hormonal imbalance (lower gut hormone despite eating) and subject keeps eating. Then there's binge eaters. Each of these three types of over-eating have a different "neurochemical hunger/stress response" to a normal person who has eaten enough and stops. [0]
[0] BBC Food, Gabriel Weston MDBS, Truth about Fat ~ https://www.youtube.com/watch?v=e9dDlKI623E