Theros: Pharika probably was Xanthe's first attempt, and (for one reason or another) it didn't work out. Perhaps it required a medicinal regimen that was difficult to produce and unpleasant to undergo. Perhaps there were side effects, like say skin molting, that nauseated and disgusted Xanthe into abandoning that method. Perhaps Pharika found Xanthe too... much of something She didn't like (too impetuous, perhaps, or too arrogant), and refused to pass along the secret of immortality.
Tarkir: Have the Dragons timeline be similar, but have the ainok be a slave to Dromoka or Atarka. That will help fuel the desire to learn hiding magic, to avoid the wrath of the master.
Awesomely put. Just wonder A. How the contract actually may have worked B. How the contract and/or demonic enchantment aspect functions for her on Theros. Is the Nyx effect more sinister and "underworldly" in nature due to her transformation being "artificial" thanks to the demon bestowing it upon her? Is the starry appearance essentially a visual cue of the contract and/or physical presence of the demon upon her body/within her soul similar to Liliana's magical scars?
And C. Deck-wise, Morph/Manifest/Megamorph primarily fall into Blue & Green with a splash of White... but Ainok seeeem to be primarily Green, Abzan sand magic seems to be mostly White (but with
Ancestral Vengeance and
Duneblast seems Black is sensible too... and the Ainok breeds found in Temur territory are mountainous and are all Red. While unsure about maximum colors in the representative deck, I'd prefer the Ainok character to be Green for sure... and idk I guess the Blue magic is just coincidental, that while the "morph" magic itself is colorless in nature, the cards that would best interact with those types of effects would be Blue followed by Green. So, perhaps a Naya-colored Ainok Archer...Ranger type character (appropriate, as more Ainok are Green than any color, mountainous Ainok in Temur are Red while none are Blue, and sand magic is White and Abzan territory Ainok are White but none are Black. Only issue I feel is, without Blue magic in his repetoire, that makes snow-elemental magic pretty much moot lol Unless I use logic from Ice Age/Cold Snap with Snow-Covered Mountains xD Or just let it be known he's Green & Red via heritage, but Blue and White via magical prowess (no pun intended)
Best interpretation could be he's Mono-Green as an Ainok Ranger and splashes other colors in a druidic way based on terrain and kind of on heritage. Splash red for mountainous terrain and his bear-like ainok blood, splash white for desert sands and hills and his jackal-like ainok blood, splash blue for ice and snow and for sharpening his morph magic, splash black for swamplands, bogs, and some of the darker needs of camouflage magic.