How can one be certain that their repentance has been accepted, especially regarding grievances that cannot be redressed and grave sins that are repeatedly committed? And is it possible to meet Allah free from sins despite repeatedly returning to them?

The repentant is beloved by Allah the Almighty, for Allah says: "Indeed, Allah loves those who repent."

A Muslim should not despair of Allah's mercy, for no matter how great their sins, they are insignificant before Allah's pardon and His mercy, which encompasses all things.

As for the material rights of people, it is obligatory to return them or seek forgiveness from their owners. If this is not possible, then Allah will take care of them on the Day of Judgment if the repentant is sincere.

As for non-material rights, such as backbiting and slander, the preponderant opinion is that seeking forgiveness (istighfar) and supplicating for those wronged is sufficient.