What I don't understand (and I am going to regret this)
Why will you regret that question?
is how some can argue that calorie deficit is the only necessity but at the same time blame a particular approach to nutrition for the obesity epidemic.
Your question has this backwards. I'm not claiming that calorie deficit is the only necessity, and I am blaming a specific approach to nutrition for the obesity epidemic.
It's those who are claiming calorie deficit is the only necessity who are not blaming any specific approach.
Let's say you ate a perfectly energy balanced and nutritionally balanced diet. And suppose someone followed you around and gave you a dose of insulin after every meal, enough to lower your blood sugar by, say 10 points. What do you think would happen?
The extra insulin would force your body to store fat and would prevent your body from using stored fat. You would not have enough fuel to metabolize, so your body would signal you to bring in more energy (hunger) and would decrease your energy output (lethargy). Do that for a few years and, hey presto! obesity.
That's exactly what happens on a diet high in carbs, with significant fast simple carbs (sugars; processed flour; pasta). It is exactly the same effect. The response to the blood glucose spice that comes from a high carb diet with significant simple carbs is just like an injection of insulin.
On any nutrition plan, if you eat more than you use - you look like me.
Or Arnold Schwartzeneggar. Or Kareem Abdul Jabaar. The difference is that some of us store the extra energy in muscle and other lean tissue and some store it in fat.
The worst part of the obesity epidemic, by the way, is the fact that it's now hitting children harder than adults. Young adults, teenagers, pre-teens, grade schoolers, pre-schooler, toddlers, infants, even newborns.
Something has changed in the environment to cause this, and the only plausible answer is diet. The knee-jerk response is "energy surplus" or "calories in, calories out" but children, especially newborns, infants and toddlers, eat when they're hungry stop when they're full and they ate that way throughout human history up to today.
I have some colleagues and family members who have found success with the fasting mimicking diet (including one who has reversed a diabetes diagnosis). I neither endorse or reject this approach.
I'm curious are you referring to the patented diet referred to in OP, or are you referring to intermittent fasting or one of the other diets that's been around for some time?
I have tried various approaches, the things that I find most difficult/impractical, is portion management and dining out (especially when calorie counting).
I'm guessing you haven't tried a LCHF Ketogenic diet. (Saying that, there are some that claim to be LCHF, but don't reduce carbs nearly enough for ketosis and allow too many simple carbs in the food mix.)
One advantage to that form of a weight loss diet is you eat when you're hungry, whenever you're hungry, and keep eating until you're full. You don't worry about portion control.
Exercise is the easy bit (Additionally, I suspect, based on short term experiements, my metabolic rate is much lower than expected by calculators in the vicinity of 400cal per day).
Right, but exercise doesn't help with weight loss much, if at all. (Though it does have other benefits)
As for your basal metabolic rate, it could well be off by that much. As in most of nutrition, it's not an exact science.