I feel your approach is reasonably sound Oz . . . simply encouraging her to use critical thinking . . . on any matter.
I always avoid the term "critical thinking" as the label is used often in XJW circles . . . but you can still teach her the process.
She is rapidly taking ownership of her own mind and decision making at that age, albeit inexperienced and vulnerable to cult indoctrination, and her eventual decision regarding the JW's will have to be a product of her own conclusions.
Direct questioning of JW teaching and beliefs is fraught with danger . . . teaching her to have a questioning mind however, is more likely to keep her closer and bring about a favorable result in the end.
Well, that's my feelings anyway . . . keep doing what you're doing in a way that is not too full on . . . cos you're right, her future is at stake.
All the best with her.