What is required of the heirs regarding their lack of knowledge about the account books of their agent for their money, and will they be held accountable for that on the Day of Judgment, especially since some of the heirs have passed away?

The trustee must be honest, giving everyone their due right, and not giving anyone more than their right except with the consent of all heirs. If he falls short in trustworthiness, his reckoning is with Allah, for martyrdom in the cause of Allah does not expiate a breach of trust. Whoever takes another's right must return it or seek the owner's forgiveness. If he dies before returning it, he will render it on the Day of Resurrection, when rights are restored to their rightful owners.