When one appeals to authority, it's better if those authorities exist and actually say anything related to one's argument. You haven't addressed my point, you cite
yourself and you should be citing ecology rather than anthropology anyway. I
have read some anthropology (Graeber and Wingrow, which you cite in note 2, is within arm's reach right now, which is why I know your citation doesn't appear on the page you list or any nearby page I can find, nor does it appear in the other document you link for that sentence in note 3 [0]), and IME in anthropology papers on a fairly regular basis one sees howlers like this one:
The Quaternary Megafauna Extinction (QME) was the well documented, extremely rapid crash in available calories, beginning around 50,000 BC, and accelerating into the Neolithic around 11,000 BC.
We don't need to wade through the literally contradictory citations noted for this proposition; it's ridiculous enough to reject on sight. 39,000 years is "extremely rapid" on a cosmic scale but not on a human one. It's not as though big game hunters were flying airplanes around the globe to collect trophies. People in e.g. northern Europe didn't share anything cultural with people in e.g. southeast Asia. Humans responded to ecological reality, but it most likely did not take the form that you claim.
It's easy to convince yourself of this. In our time, modern agriculture and modern chemical manufacturing provide food for most of us, but not all. In remote locations throughout the world, right now but more significantly within the last millennium, indigenous neolithic populations support and have supported themselves in "the old ways". That is, even though no giant ground sloths exist (although large mammals such as buffalo, elk, moose, elephant, hippopotamus, rhinoceros, giraffe, etc. do) and even though modern humans have been consuming most global resources, these isolated tribes have successfully fed themselves. They have been visited by anthropologists who have recorded their lifestyles of e.g. monkey hunting, clam digging, or swidden agriculture. They don't starve and they don't hoard. In most locations, meat makes up less than half of the diet. Foraging plants and consuming small game isn't strange and isn't inefficient. That is what hominids have done throughout our existence.
As we know from ecology, the particular species that make up a trophic level do not change the fact that energy flows up the food chain. Perhaps koalas might die out if eucalyptus disappears, but humans are highly creative and adaptable to various food baskets. Your speculative, ecologically unlikely argument needs more support than an "imagine you're a nomad" just-so story. For one thing, we "evolved" to eat fruit not mastodons. Some of us have eaten mastodon when it has been available, but it has only ever been available on a small portion of earth's landmass. When mastodons disappeared that probably caused some people to starve but starvation is a common cause of death for any predator. That's why humans in the state of nature typically eat plants that they forage: to avoid starvation.
I mostly agree with the "against the grain" hypothesis, but it has been covered better elsewhere. I haven't spent enough time thinking about your later propositions to judge them. This paper may be recoverable with some editing.
[0] http://pacificecologist.org/archive/18/pe18-hunter-gatherers...