We already see many NPC's readily working alongside defectors from Garlemald and the major organisations that our character works with do not discriminate based on race or background. Breaking the cycle of distrust and conflict is...the major plot element of a great many storylines throughout the entirety of the game thus far.
They didn't add any special dialogue for other race and gender combinations added to the game at a point later than ARR. Thus they wouldn't need to do as much in the case of a hypothetical Pureblood Garlean option.
I respectfully disagree. I don't think it'd be jarring at all and I believe it to be a subjective element at play - nor at any point are Pureblood Garleans or Garlean defectors shown to be arrested or hunted down in the streets if they are not aligned with Imperial Garlemald. We also know that the Warrior of Light is effectively a thrill seeker and mercenary for hire on many fronts, often with little ties to whatever nation he or she hails from. It also doesn't take very long for the player character to hold significant sway over multiple world leaders/prominent figures...so I doubt the fact that the player character is a Xaela or a Pureblood Garlean or a Hyur or a Viera is going to matter much to them.
Given that the majority of Eorzeans are canonically incapable of reading or writing there's a pretty good chance that they wouldn't know what a Pureblood Garlean even looked like - to say nothing of the fact that a third eye is easily hidden through the use of strategic hair placement, headgear or via a glamour.
Putting aside the fact that every request thread needs to start somewhere, I think it is rather disingenuous - perhaps unintentionally so - to imply that there's no demand for Pureblood Garleans as a playable race at the same time as actively shutting down attempts to push for them to be added to the game.
Indeed, one might wonder if the real opposition comes from a fear that the request might actually gather some traction and come to be implemented in-game. We also know that popularity alone doesn't always equal whether something will be added to the game or not.
There's also a lot of fans of Garlemald and role-players who already choose to portray their characters as being Pureblood Garleans. Perhaps a search for 'Garlean' on Twitter alongside the FFXIV tag would be a good start to get a quick and easy glimpse at that. I know a lot of them don't bother posting on the official forums, though - because they tend to tire of the usual song and dance that usually arises where they're told they're wrong to like or support the 'bad guys' even though the game's writers have confirmed multiple times in interviews that the game world isn't intended to be taken in the context of 'good' and 'evil' but merely a nuanced, complex matter of perspective. I imagine that reflects in the views of the general populace, too - and if you'd like, I'm happy to find and link those interviews here.
Come to think of it, a playable Pureblood Garlean would be very much in line with that premise. A lot of people also enjoy playing a character with a lot to prove or with the odds stacked against them, too. I could see it drawing a lot of new players to the game - I always felt like there were a lot of story similarities between Pureblood Garleans and the Blood Elves of WoW for example and I believe there's been quite the influx of WoW players checking out FFXIV lately.
For what it's worth, I could understand the story not making sense if people were asking for playable Pureblood Garleans who are still fighting under the banner of Imperial Garlemald and proclaiming their superiority over the rest of Eorzea but that isn't what is being asked for here.
Ultimately, though, the prudent course would be to agree to disagree I believe.