How can one repent of sins such as backbiting and theft if one has forgotten some of those whose rights were violated, or fears informing them, or does not know their whereabouts? And can the stolen amounts be given in charity instead of being returned to their owners if they are insignificant and there is no fear of being asked to return them?

All praise is due to Allah, who has accepted your repentance. The rights of people must be fulfilled. Therefore, stolen goods must be returned to their owners or their forgiveness sought, without informing them of the theft. As for backbiting, it is sufficient to seek forgiveness for the one you backbit about and to supplicate for good for them, in order to avoid the harms that would result from informing them.