Meh, let her take the blame at this point. She fails Darwinism. She's the kind of person who drags herself down and drags everyone attached to her down with her. Don't let her drag you down with her. You can still find the real culprit and eliminate that threat without her.
If I were her father I'd make it clear that I won't be sacrificing my daughter to the public's betrayal nor to satisfy an imposter. If the public pushes, I'd sell out my own kingdom to its enemies and help them destroy my own people. You attack me, especially my family, I return it in kind. After the dust settles, I'd happily rub in their faces that there weren't any more monster attacks, especially if the culprit was caught. Maybe I'd punish the public on the grounds that "If there's no more monster attacks after you were crushed by an army, that means YOU were the real culprits, so you should be punished for that." I'd make them dearly regret their betrayal.