My thoughts on the food taboo:
I think it’s possible the fairy food taboo has its roots in h. sapiens’ interactions with other Homo species.
My understanding of ancient human evolution in Africa is that it’s likely there were several h. Erectus subspecies along with archaic h. Sapiens for a significant period of time. Also, it’s fairly likely we’ve had interactions with h. Floriensis in Asia. In addition, we know we’ve interacted with Denisovians, Neanderthals, and a third, as yet unknown Homo species. If you want to stretch the point, you would likely have groups of h. Sapiens that are different levels of development running into each other.
To me, it seems reasonable that at least some of these other human species would probably trip our “uncanny valley” reaction, or at least be surrounded by wonder and oddness. At the least, you have the human tendency to assign supernaturalness to the “other.” It’s likely h. Erectus preyed on modern humans from time to time, but we also may have had non violent interactions as well.
Including interbreeding. Perhaps breeding with h. Erectus, denisovians, etc., was taboo, and the fairy stories are a hold over of that. Sharing resources or living with these groups may have been a no-no too. Remember, it seems that having only one Homo species around is not the norm, which means we’ve encountered populations of what we may have considered “nearly people” since we evolved.
EDIT: tl;dr- Maybe fairy myths are The remnants of our social norms governing how we interact with oth species of the Homo genus