I think the position of her feet, which you don’t mention, bear your point out: her right foot faces the viewer but the left foot points away from the lyre player. All the implied momentum of her body as she turns is about turning away from the young man, including her bent right arm, which implies a kind of gentle defense, and her left hand holding the lyre, mirroring the left foot bent on escape. In other words, I think she is about to turn full-frontally toward the viewer, as you say, but only briefly as she rotates completely away from him (back to the feet: his, facing right at her, indicate total engagement; hers, complete ambivalence and the start of flight).