[Belegorn]

Tar that is interesting. It appears to fly in the face of everything that has been attributed to Fëanor. Look at the first quote for instance. I would assume that Lúthien was one of the Children of Iluvatar. There is a contradiction there.

"Fëanor was made the mightiest in all parts of body and mind... of all the Children of Iluvatar" [Sil, p. 112]

There are instances where terms like these, greatest, mightiest, or fairest have been used for various characters. I'd ask if it's this one or that?

Contradiction aside, whether Luthien, or Galadriel, I would say that Sauron or some other great force of Melkor's could take down any realm in the F.A. held by different bearers of the Rings. The Elves are powerful and even the really top one's like Glorfindel, who is said to be nearly equal to the Maiar, could not stop them. If he is held to be so powerful, and Galadriel who is greater imo as per, "the mightiest and fairest of all the elves that remained in Middle-earth" [Sil, p. 370], could have her realm toppled by a Ringless Sauron if he chose to throw all of his strength at her, then any other Elven kingdom could be destroyed no matter what Elf held a Ring of Power.