I guess we can agree to disagree on that.

There's a lot of Pureblood Garleans out there. Perhaps some of them acquired the Echo, which is a prerequisite for becoming a Warrior of Light. I don't think it's any more of a stretch for a Pureblood Garlean to set off on an adventure just like a Xaela taking their leave of the Steppe or a Viera departing their forest to do the same thing.

A clear distinction is also made between Imperial Garleans and regular Garleans and the inhabitants of Eorzea do not show immense hostility towards the latter. Indeed, we've seen our character and the Scions work with them many times already. Maxima, Cid and Nero spring to mind but there's other examples as well.

Personally I'd be happy with a third eye glamour, since you can design a pretty neat looking 'Garlean' using either a Midlander or Highlander model already. As many Garlean role-players already do.

I just don't think the lore is set up in such a way as to make it impossible for a Pureblood Garlean to exist as a playable option.