The princess' twist isn't very surprising. She clearly has some knowledge of future events (and might be a time traveler herself, or have interacted with another traveler prior to FL) and wants to act according to them. If you haven't forgotten, a couple of chapters ago she mentioned she could use the built-up magic from the GMG to release it into an Etherion of the sorts (which is why I think she's waiting until the end of the games to do something).
We all know using a superweapon to solve a problem always backfires, so there's something wrong with that. The princess could very well not be lying, either, but simply have a different outlook of the future.
Or even, her perception was manipulated so that she's led to believe using the Etherionish power is the only way to stop the dragons. Oooooh.