What must I do regarding rights I have not returned to their owners after repenting from sins and transgressions, especially if the owners of these rights are unknown or difficult to reach? And does the rule "Islam nullifies what came before it" apply to me concerning the rights of Allah and the rights of people?

Repentance from transgressing the rights of human beings through theft or similar acts is achieved by seeking their forgiveness or by returning the rights to their owners if possible. If that is not possible due to not knowing the owners, then it is sufficient to give charity on their behalf. As for repentance from backbiting, some scholars hold that it is achieved by seeking forgiveness for the one who was backbited, supplicating for them, and mentioning them positively, along with sincere repentance.