I've also discovered that it is possible to ride ALL 4 Fantasyland dark rides in under 15 minutes immediately after the all clear to open Fantasyland is given after the fireworks. We use this little gem frequently on our trips.
Also, Disney wants to lose their castle even less than we want it to be lost. After all, they have to do any repairs/reconstruction on it and pay for it. No... they've done their homework and risk assessments and take precautions to protect it. Any additional risk from launching fireworks off of it are minimal and probably no more than the risk that something inside it might short out and catch fire (lighting, projectors, etc for the walkthrough).