Arwen's friend should be one of the Noldor living in Rivendell, so her name should fit that. (And I agree on saving Nellas, as we may want to actually cast her.) I would suggest Olótië (means Blossom, IIRC).
No, you're right, late 3rd age should definitely be a Sindarin name, I just haven't had the chance to translate it yet or pick one.
'talagand' means 'harper' - we can show her playing a harp at the festival where she is introduced, and name her some variation of that. Alternately, we can name her Eirien (Daisy).