Three Eye granted Sight- veil destroying Sight- to mortals.
Not quite as simple as that; it was addictive, and people only got the Sight while they were high (and borderline irrational). "Everybody knows" you can't believe what's seen on a hallucinatory high. So this channel is inherently discredited -- from the muggle POV -- from the beginning.
Even Harry didn't think it was anything other than "just another street-drug," until one of the addicts called out HWWB.
If you already know about the supernatural and the Sight -- if you're already a practitioner -- then it's relatively easy to confirm the validity of ThreeEye; but virtually impossible, for Muggles to even
want to do so, or to try; let alone to
succeed.
As an "unmask the masquerade" project, ThreeEye is kind of a non-starter.
The FBI were trying to expose the loup garou to hide their own activities.
Again, I think there's more complexity here: The FBI agents were pretty wide-ranging in scope, and couldn't "cover their tracks" with a single Loup Garou in Chicago. They were just taking advantage of the specifics of this
particular case to muddy the waters a bit. But a "wolf" isn't going to "expose the supernatural" -- it's just a wolf!
Truth, I also think Lord Raith was the primary antagonist through the first few books
(Also, of course, Jim was writing "the werewolf book" and wanted multiple
kinds of werewolf.)
And Kravos was empowering ghost appearances with far more power.
Except ghosts -- like hallucinogenic drugs -- are an inherently non-credible channel to expose the supernatural. The muggles already have their built-in excuses and firmly-worn blinkers.
Truth, I also think Lord Raith was the primary antagonist through the first few books
I think you are right about this; but then, I think that weakens your main premise -- the Whamps would
hate to have a widespread awareness of the supernatural amongst the mortals!