Does throwing the pebbles before dawn on the Day of Sacrifice by two hours or less necessitate a دم (expiatory sacrifice), and how can I rectify that?

If you are referring to Jamrat al-Aqabah on the Day of Sacrifice (Yawm al-Nahr), it is permissible to stone it from the middle of the night of Eid according to many jurists, such as the Shafi'is and Hanbalis. Therefore, no expiatory sacrifice (dam) is due from you if you stoned it two hours before Fajr. As for the strong person, it is better to delay the stoning until sunrise, following the example of the Prophet (peace and blessings of Allah be upon him).