I like this very much--if Elrond has a meaningful relationship with Maglor as an adult, despite everything, I think this is what it would look like. You write their interactions in a very understated way, but the affection between them is obvious and believable; and I like the reason you give for Maglor's exile, part choice and part compulsion.

I also really like the glimpse you give of Tidir, and her dictionary--it's a lovely bit of world-building.