Is it obligatory to return stolen items to their known owners, or to donate them on their behalf if they are unknown or difficult to reach? And does the absence of feeling guilty about past thefts—due to a lack of religious guidance at the time—affect the validity of sincere repentance?

That which you stole before puberty carries no sin, as the pen is lifted from the minor. However, you must guarantee its return to its rightful owners. If it is possible to convey the right to its owner, such as by directly or indirectly returning the value of the stolen items to them, then do so. If it is impossible to convey it to them, then it should be given as charity on their behalf.

As for a non-belligerent disbeliever, their right must be rendered to them. The moral corruption of parents does not nullify the duty to honor them, and their companionship must be maintained with kindness, but there is no obedience to a creature in disobedience to the Creator.

Repentance involves feeling guilt and remorse, and Allah accepts the repentance of those who repent.