No they wouldn’t stay lean. If you fed someone more calories than their body uses to maintain its current weight, they would get fat eventually. Regardless of whether they eat carbs, fat or protein.
Let's say you're being fed just the amount of calories your body needs to maintain. What influence on body fat has the distribution of carbs, protein and fat from food then?
My point was that regardless of what you eat, if you eat more than your body expends, some of the excess will be used to produce fat and you will gain weight. If you lead a sedentary lifestyle and eat 3000 calories of nothing but lean meat per day, you will gain weight.
You are right that different foods (macronutrients, more specifically) have different thermic effects and therefore require different amounts of energy to metabolise. Protein takes more energy to process than fat. This does not change my overall point.
Not much for getting lean but it's easier to overeat carbs and fats are calorie dense. That said you need to eat a mix of the three to be healthy because they all have important body functions.
You're so wrong. With only protein and fat, they would not be able to gain much weight. I've tried to gain weight on low carb/high fat, and it doesn't work. I was uncomfortably full after every meal. My tracking showed a significant calorie surplus, but my body wasn't absorbing it. Adding in more carbs made a huge difference. Hormones tell your body when to store fat.
Did you know that bodybuilders in the 1950's(before anabolic steroids) used the egg and steak diet before contests as cutting diet to get lean? They would eat, as the name suggests, large quantities of steak and eggs with butter and have a carb refeeding day every 4 days. They would get really lean on a very high-calorie diet, think 4000 to 6000 calories.
The inventor of diet famously lost a contest for being too lean.