Is mentioning an incident in which the grandmother was wronged by a deceased person, while praying for his forgiveness and mercy, considered backbiting, and how can one atone for that if it was backbiting?

Mentioning that a person is unjust is considered backbiting, because it is something that would displease him. And backbiting about the deceased is worse than backbiting about the living, unless mentioning him negatively serves a benefit, such as warning against his innovation. If what you said was unintentional, then you are not held accountable, because the Prophet, peace and blessings be upon him, said: "Indeed, Allah has forgiven my Ummah for their mistakes, their forgetfulness, and what they are forced to do."