The AI's last answer in "Plan my book with me" made things worse. Can I undo it?

Yes, from version 1.9.24. Above the proposal, press "Undo this answer". It puts the proposal back exactly as it was before your last request, your own changes included, and your request goes back into the box so you can word it differently. You can undo several answers in a row. Once you agree to a stage, its answers can no longer be undone.