Is the fiancée sinful if she supplicates for Allah to turn the plot of whoever plots against her back upon him?

Supplicating with this phrasing is permissible. The fundamental principle regarding a Muslim is their integrity, and one must have a good opinion of them unless suspicion arises. Their words and actions should be interpreted in the best possible light. Therefore, it is not permissible to have a bad opinion of a suitor based merely on doubts and delusions.