How can the narration of Uthman ibn Affan (may Allah be pleased with him), from which it is understood that the questioning of the deceased occurs after the completion of his burial, be reconciled with the narration in Bukhari, from which it is understood that the questioning of the deceased occurs after people have departed from him?

There is no contradiction between the hadiths that discuss the questioning of the deceased in their grave. Both hadiths indicate that the deceased is questioned in their grave immediately after burial. It is prescribed to seek forgiveness for the deceased and ask for their steadfastness upon completion of their burial because they are being questioned at that very moment. If some people depart while others remain seeking forgiveness for the deceased, the deceased hears the sound of the departing people's footsteps and benefits from the forgiveness of those seeking it, at the same time they are being questioned.