In the end, Billy ends up making a new mirror, and at night, he goes toward the nearest dungeon. After summoning and making some monsters hunt the frozen wolves by themselves, Billy began to recover mana pretty fast. He infused that mana into the mirror and made it move toward the past. A shit ton of mana was used, but he managed to finally see the time when Haruna's parents were alive when the sun was about to rise.
They stayed in their beds for about two months, unable to do much. Billy tried to check their status, but he was unable to use Appraisal on things that the mirror was showing. In any case, after a couple of hours, Billy confirmed that no one around them made them drink or eat anything suspicious… If they hadn't been poisoned, perhaps they had been Cursed, but Billy had no indications of that yet.