I've always seen Posy as having a hefty dose of horse-sense. There are times when she's sentimental, but I've always thought of her as having that roll-up-her-sleeves and get the job done type of mentality. I think that is what has sustained her through this.
For a girl whose parents had her bags all packed (see post #1), she's got a lot of self confidence, too. She's kind of irrepressible, which is good for the only gnome, and only girl in a party with strong-willed dwarves.
As for her innocence, well, I think you are on target, but there is a part of Posy that is older than her years. She's very mature about inter-personal matters, and has good insights. Her familiar, Ember the dog, (+2 Sense Motive) has helped her with that also.
Oghma