Will the deceased's parents be guaranteed [the reward for] what they did by donating all the money, despite their ignorance of the obligation to pay Zakat and the amount of the Nisab, and their not intending for a part of it to be Zakat or expiation?

Your deceased parents are not obligated to guarantee what they gave in charity, whether they gave it on their own behalf or on behalf of the deceased and did not intend it as zakat (definition). This is because the issue of paying zakat from the deceased's wealth after their death is not a matter of consensus among scholars. The Hanafis held that it lapses with death, as it is an act of worship conditional on intention. If it was a voluntary charity, then it benefits the deceased and will complete any deficient zakat on the Day of Resurrection.