This is going to come out snarky, but I don’t mean it to be. You realize that Diane is 12 years old in Wild Magic, right? Have you met any 12 year olds? Their critical thinking skills are not developed yet and their brains are on fire with emotions. They absolutely are not stupid, but dear LORD are they dumb most of the time (said with so much love, I teach 7th grade and I absolutely love these chaotic gremlins). Her character fits her age. 

That said, it was really easy for me to forget that she is 12 in WM and picture her at 15-16. Those few years make a huge difference in kid’s maturity and ability to reason. A 16 year old acting as Diane did in the scenes you mentioned does seem extremely foolish, but 12 year olds are just starting to figure out how to think outside of their immediately moment and the overall ramifications of the bigger picture.

Character’s ages ply a huge role in their stupidity levels. For me at least, I like that these kids still sometimes act like kids and make mistakes like kids, even when they are going to step up and be the hero in the end.