Unfortunately it’s a nature of the game having multiple writers and being over a decade old that the game’s lore even sometimes contradicts itself now. I wouldn’t trust a lot of RP sites because that’s where a lot of really weird misinformation is created and spreads like wildfire. There’s this one Tumblr blog I really like because the person behind it has categorized everything really nicely and they took the time to source everything and it’s the exact text from the game or from an interview without a filter over it.

In regards to the Garlean third eye, it’s supposed to be an organ that greatly increases their spacial recognition making them better at flying airships and shooting guns. It’s also supposed to be hard like keratin. Only pureblood Garleans are supposed to have it, and 3 half-Garlean NPCs are definitely missing it, but a half-Garlean in the Werlyt quest line randomly has one so I guess that’s thrown out the window now.