How is the riddle a clever summary of the whole novel?

Becoming his own hero also means taking responsibility for his own actions. In the final scene with the Brazen Head, Dunstan comprehends that he was as much a part of Percy’s death as Paul was. Liesl had connected Dunstan with his dark side, the other half of his personality that would make him whole again. In effect, she taught him to accept the devil, much as Padre Blazon encourages him to do in this section. The fact that she is the voice of the Brazen Head only underscores her importance in reminding him that he has the devil inside him.

The heart attack is not only a recognition that he let the devil in through his interference, but also a chance to be born again. He has already been reborn - from Dunstable to Dunstan - but here becomes even more full, a human who can reach his full potential by accepting his contradictions. He then consciously accepts this new identity by accepting Liesl’s invitation to join the carnival in Switzerland.