Some really interesting stuff here.
I'll be honest, I always find the 'humans have evolved to be omnivorous' argument a little unconvincing. In saying that I'm not dismissing the obvious fact that we can eat and digest animal-based food in addition to plant-based nutrition.
Rather I believe humans have more or less 'guided' our evolutionary path with respect to our diet to have arrived at our current state of omnivorousness.
Without a doubt in my opinion, the over-riding evolutionary impetus in our species' history was our mastery of fire. Ignoring for this discussion the obvious heating and animal-repelling advantages of control of fire, important as they were, I believe the most significant benefit was the use of fire to cook.
Cooking food, aside from (usually) making it taste nicer (although it could be argued that cooked food tastes nice because of evolutionary pressures), also makes it more nutritious and more digestible than (pound-for-pound) the raw ingredients.
Another massive advantage that cooking their food conferred on early humans was an incredible saving of time and to a lesser extent, effort. This benefit manifested itself in two ways; firstly that requiring lesser amounts of food to obtain the same nutritional value meant less time spent hunting and gathering, and secondly that cooked food can also be eaten and digested much more quickly than most raw food.
I would argue that current humans (especially those in the west) wouldn't be that well adapted to consuming raw meat; not just because we would need to eat a lot of it but that it would take us hours to physically masticate and swallow it. Hence my initial statement regarding our omnivorousness.