[Belegorn]

I'm not sure where Luthien is said to be the greatest Elf but it does appear that Tolkien does use the same descriptions for various characters. Like two different characters might have "strongest" alotted to them.

I would not think Luthien was inherently more powerful than Feanor. From my reading several different texts state that Feanor was the supreme Elf. For example, in Morgoth's Ring it's said of Feanor, "This child is the greatest in gifts that hath arisen or shall arise among the Eldar." [240] Greatest in gifts which is quite specific. Mandos says, "Aule nameth Feanor the greatest of the Eldar, and in potency that is true." [247]