Is there lore for a half elf female to be something like such a lightly armored sword dancer?
Second question, how/wherefrom do the enemies get the damage when they miss you?
Yup, any type of elf/half-elf can be Eilistraee's sword dancer. As for the damage it's from
Stage Fright, an Illithid ability.
Paired with the singing [Resonance Stone] which makes everyone around her "happier" (vulnerable to Psychic damage), since it is her secondary source of damage and fits the roleplay of her song soothing the enemies around her while dancing. Which is why she's also wearing the [Strange Conduit Ring] and [Braindrain Gloves], along with [The Band of the Mystic Scoundrel] so she can cast Vicious Mockery too while swinging her blade.