Could a Human Being simply be taught to become a shape shifter?
More like an illusionist, probably - capable of changing the way he appears to the audience, but not actual physical measurements, not the way Odo can squeeze through a keyhole or Martia can escape from leg irons.
Kirk had already encountered such an illusionist, the Salt Vampire. If Garth falls in the same category, then Kirk's apparent amazement at Martia's ability to physically change shape in ST6 becomes more understandable.
TOS has already established that certain Mandrake the Magician suggestive powers are available in prescription drug form for regular humans: the Venus Drug makes you so self-assured everybody thinks you are irresistible and the most beautiful thing on or off Earth, or whatever. The Antosian cure might be similar, just a tad more extensive...