The only monkey wrench in the problem with saying Coin Song is Sabin is that if you don't talk to the Matron then you do hear "Edgar and Sabin" when you first physically meet him on Mt. Kolts. This kinda still keeps Terra and Celes unique as both do not have their actual themes played when you actually physically meet them. If my memory serves me correct, doesn't Awakening sometimes play when Terra isn't even present?

Really, Terra's theme is everywhere in the VI soundtrack cause its used as a leitmotif in Awakening, Metamorphosis (a track that is really more associated with Espers than Terra)as well as the Opening and Ending themes. I feel Terra as the first character the player uses and the one (due to amnesia) that the player can relate to makes her a bit of the main character in the first part of the game so it does make sense the world map theme is also her theme (personally I think Cloud's theme in VII is the world map theme as well). Often times I feel the themes are also played just to represent the theme of what's happening in the story.

Granted I do feel Awakening should be her theme as its easily the best version of her theme and the one that I feel really does express her personality. I think where I might disagree with you is that I feel Coin Song is just as much about Edgar as it is about Sabin but being twins it is hard to think of them as separate entities sometimes.