Awww, the unbuttoning bit sounds really cute! I think it's cool that you're doing it freehand. And yay for starting on the next character soon!

Hmm, I would maybe do it as a pendant? You can't pin a brooch to a hard shell (although I guess you could attach it some other way). I mean, I know the shell is really made of yarn, but in the story it's hard, right? So it might look strange if it was a brooch.