WI Grimm's fairy tales are based on real, living human subspecies that went extinct centuries ago.
WI Saquach and Yeti are based on tales of Neanderthals or Denisovians?
WI elves are based on Williams Syndrome gene disease?
WI leprechauns are based on Donoghue Syndrome?
What's the big "what if" here? The idea that William's Syndrome influenced the popular conception of elves, especially since the Romantic period, is very possibly reality. The same is true for legends of changelings and "feral children" being influenced by pre-modern observations of autism and so on. If true, it doesn't really change anything at all.
Your description of Grimm's fairy tales seems to be mixed up with Scandinavian folklore - Not as many Tolkienesque creatures lurking around in the Grimm collections as you think. A lot of what exists in European folklore, including elves, fairies, kobolds, gnomes, nymphs, trolls, trasgu, brownies, domovoi,
pixies, lutin, ellyllon, nisse, and so on, seem to be survivals of pre-Christian religions, in a parallel to the kami of Shintoism, the lares of the ancient Romans, or any number of spirits found in other folk religions. Some may have evolved from ancestor worship while others might be hearth gods reappropriated. When I read descriptions of pre-modern Germans setting aside spots on the hearth for kobolds or pre-industrial Scots offering wheat or milk or ale to brownies, I can't help but think of the Cai Shen statues in Chinese houses and shops along with the oranges and apples set before them.