Fail to see how a Pureblood, who could arguably hide away their third eye with magic glamours, is more of an issue than any of the above. And in case being nearby one with their third eye on display breaks muh immersion, how about a WoL in a chicken suit?
Read that carefully. It matters little if one thinks this is a "retcon" or whatever else; it is the current understanding on their ability to wield magic, and it allows for some exceptions, which the WoL could certainly fit
without any handwaving over and above the usual - all without even requiring the contrivance of the Resonance. As for the Echo? Unless Garlean Purebloods don't share the same type of soul everyone else does, they too should, in principle, be able to have the Echo awakened within them.