Does the supplication return upon the oppressor if one person supplicates against another unjustly, such as supplicating that a rock fall upon him?

Supplicating against others unjustly is often not answered. It might be answered, but the supplicant would then incur sin or be punished. There is no explicit mention in Islamic law that a supplication turns back on the supplicant, except in the case of cursing someone who does not deserve to be cursed, in which case the curse returns upon the one who uttered it. It is permissible to supplicate against a person only to the extent of their deserving. If they do not deserve the supplication, or deserve less, then the harm might rebound upon the supplicant, for the recompense is of the same nature as the deed. The angel's saying, "And to you the like of it," applies to supplicating for others with good, not with evil.