What is the penalty for one who did not expiate for several oaths whose time had passed before he committed to doing so, and will Allah forgive him?

If a person has incurred many expiations for oaths (kaffarat yameen), even from before their commitment (to regular religious practice), they must discharge them according to their ability. If they do not know the exact number, they should err on the side of caution until they are reasonably certain that their obligation has been fulfilled. If they die before discharging them, these expiations must be paid from their estate before its distribution and are not forfeited by death. If they are not paid from their estate, or if they left no assets to cover them, then their matter is with Allah Almighty; He may forgive them if He wills, or punish them if He wills. This falls under unseen matters about which no definitive statement can be made.