If you ask me then I would say "Awakening" and "Coin Song" are Terra's and Sabin's respective theme.
There's no actual reason to doubt that either, because why would Overworld Theme necessarily have to be Terra's "real" theme?
Might as well be the one that plays during her introduction, otherwise you could say that "Searching For Friends" is Setzer's theme or something.
And "Coin Song" is very similar in melody to Edgar's Theme, so it would make sense that Edgar's twin brother has that as his own theme - it even expresses that sorrow Sabin probably felt when he left.