Is it true that Allah is the Transformer of hearts, and that what happened—the refusal of marriage after the engagement and the recitation of Al-Fatiha by the young man and his family, after an agreement had been reached—was destiny? And am I entitled to know the reason for this sudden change after all we have been through?

There is no harm in supplicating that Allah makes this young man your destined partner. Even better is to supplicate that Allah chooses what is best for you, wherever it may be. You must also be mindful of Allah (have taqwa of Him), for nothing eases and facilitates matters like taqwa. Regarding any material or moral harm that resulted from the suitor's actions against you, you have the right to sue him for it and demand your rights from him. However, we advise you to pardon and forgive, seeking your reward from Allah the Almighty.