Warning tl;dr!
Anyway, whether eating meat was evolutionary required/beneficial for brain developement isn't an argument pro/contra meat eating anyway, because obviously nowdays we do no longer have problems with our energy managment (actually we do have, but in the other direction; just look at obesity rates...).
All of your discussion seems to have been on energy requirements, but that has very little to do with the original need for meat, or at least animal products in the diet.
The big issue is the protein requirements necessary for all animal life. Proteins are made from amino acids and there are 20 amino acids needed by humans for heath development, including brain growth.
Essential and Non-Essential Amino Acids
Any time you consume protein, your body receives a source of amino acids. Your body uses combinations of 20 distinct amino acids to make up the protein in your cells. Dietary amino acids fall into two categories: non-essential amino acids, which your body can produce on its own, and essential amino acids, which your body cannot produce and therefore requires as part of your diet. Sources of protein that contain every essential amino acid are called complete proteins, while sources of protein deficient in one or more essential amino acids are incomplete proteins. Failure to eat enough of any essential amino acid prevents your body from making the protein it needs to function, leading to tissue breakdown.
Intake Recommendations
The specific amount of protein you need each day depends on your weight and activity level. Most people can calculate their required protein intake by multiplying their weight by 0.4, according to the Iowa State University Extension. For example, the average 150-pound individual would require 60 grams of protein daily. If you’re an athlete or you have a very active lifestyle, you might require more protein, up to 1.8 grams per pound. A registered dietitian can help you determine an appropriate protein intake, as well as design a diet plan to help you meet this goal.
Sources of Protein
If you follow a well-balanced and varied diet, you likely consume protein from a range of sources. Eggs, meat, dairy products, soy products and quinoa all represent sources of complete protein, while beans, lentils, nuts and whole grains contain incomplete protein. Eating a combination of foods to meet your daily protein intake helps ensure you’ll consume enough of each essential amino acid, especially if you consume some sources of complete protein. Choose lean proteins to limit your consumption of fat, and eat fatty sources of protein in moderation. Plant-based sources of protein typically contain little fat, or contain unsaturated fats that benefit your health. Fatty meats, like red meat, or full-fat dairy products contain significant amounts of saturated fat, which increases your risk of cardiovascular disease. Limit your intake of red meat, instead opting for leaner meats, such as skinless chicken or turkey breast.
From Why Are Proteins & Amino Acids Important to Life?
Energy requirements are easily (
too easily
) supplied by carbohydrates found in (over) abundance in cereals, fruit and sugars.
And they are the big problem with modern diets, though excessive fats is right up there too.
Herbivore animals still need protein and mainly have to make it from grass and other vegetation. This is usually done by bacteria and fermentation in the gut.
Horses and their relatives have very ineffecient digestive systems and need to eat a great deal of grass, or a mixed artificial diet.
On the other hand ruminants (cows, sheep, goats and the like) have multiple chambers in their digestive system allowing them to re-chew their food (chew the cud) and pass it to another chamber for bacterial action and fermentation.
Carnivores of course, get their protein from meat sources, though many including bears are omnivores.
One strange animal is the panda bear. It is a true bear and has the short gut of a carnivore, but lives on a very poor diet of almost entirely bamboo. As a result it must eat vast quantities to get sufficient nutrition.
Birds, like chickens, have a mixed diet including insects etc.
So animal products were certainly the easiest and most abundant source of protein and they provided all 20 amino acids necessary for life.
Now, where does this leave the vegetarian of vegan?
Vegetarians commonly allow animal products such as dairy products and eggs.
Then there are certain, though not all, proteins in some plant products, such as legumes where nitrogen fixing bacteria in root nodules produce it, hence the popularity of soya products.
The vegan needs to be much more careful, but there are now ways of getting the required nutrients.
The critical nutrients are those 20 amino acids, which make up the various proteins and energy requirements are a minor matter.
As far as I have seen we would certainly be better off with less animal product in our diet and that would be better if it were the likes of fish and chicken and much less red meat.