I finished the final episode 5 minutes ago, and first thought is I think this is the most likely case simply because I watched their previous work and this is what they do, they explore interesting weird ideas, keep it vague for you to decide on your own, and switch it to signal out its more grounded/real instead at the end. But ultimately completely leave it unanswered and open ended, no real conclusion in their style (in another earth and sound of my voice movies, was similar narrative style). If this is the only season, the above was their aim, to make it about symbolism and make us question or narratively doubt wether it was all true or just fantasy, finito. What is is depending on the viewers intepretation. For me IMO more leaning on it was a lie and she was kind of schizo, who happend to grow a strong bond of understanding with misfit group who got something out of being duped by her.
FBI man theory or such is just reaching it IMO (this character was the most grounded character in the series, together with her father), nothing that indicates or hints it at all, as language part, probably just a plot hole that they didnt bother to go into.
Thats my immediate impression to the fable and fantasy of it should be explored further to make this story worthwhile otherwise its pointless as you said, so it all depends on if they intended this as 1 season, or they want a 2nd. If they want a 2nd, then yeah all the existential fairytale will turn out to be proven.