Meaning of "It was a shortcut for her. She entered at ease within its complexity, at home in song and in feeling."
"Shortcut" is used metaphorically. It means, roughly, "easy way into x". And poetry can be complex, but that didn't bother her. She was comfortable with it. "At home" is another metaphor, meaning "comfortable".
The precise intended meaning of "in song and in feeling" is unclear (a bit poetic, it is), but suggests that poetry gave her "a song in her heart" (another metaphor).