Thanks to @merilles I learned about the existence of Erien who is the daughter of Fingon in a late text of Tolkien. She also has a brother named Finbor.
Mentelossë's name is her ataressë and she didn't have an amilessë because of Lotelissë being really depressed, distant from her children (and I was lazy to find another one oops). However Erien means "one/alone"+feminine suffix and I think that's the PERFECT level of mother issuesque name to give to your lonely daughter- and perfectly fits Mentelossë's story and themes.
Besides it's a sindarin name and it makes sense cause she would get her amilessë later in time: born after the Helcaraxe, gets her ataressë in quenya, quenya gets banned, gets her amilessë
IN CONCLUSION: I'M MAKING MENTELOSSË A TEXTUAL GHOST !
(Also imagine if Finbor is also Gil Galad's amilessë and Ereinion is his ataressë ... those names are soooo bad no wonder he chose to go by his epessë 😂)
