I am nearly certain that Thuringwethil was confirmed to be an Umaia, as she is presented as the first of the Vampire-Bats, in the same fashion that Draugluin is the Sire of the Werewolves, but is mentioned as being able to shapeshift to her "Bat" form (as opposed to Draugluin, who is trapped in his wolf body. (well, i admit that most of my convictions about her Maia nature are based on the many fanfics with Sauron/Thuringwethil romance back in the Timeless Halls and in Utumno and Angband. Twisted.
)
But indeed, she was more a messenger between Tol-in-Gaurhoth and Angband than a real fighter, and a diplomat role would be very appropriate (much like the Mouth of Sauron for Mordor in TATW)
And you're god damn right DieWalkure, those characters (Melian, Galadriel) never really played an active role in the War of the Jewels; the former did protect Doriath with her Girdle but she was never accounted as a fighting general - and so little is known about Galadriel's whereabouts during the First Age, I think that they would perform great as diplomats for Doriath
When it comes to Luthien however, I would see her as a spy more than a diplomat - however, as I'm sure that Bercor and his team have very special plan for Beren and the Quest of the Silmaril, I wouldn't try to guess too far