Is the grandfather's waiver of his share—one-sixth—of his deceased son's inheritance to his grandchildren considered an impediment to their obligatory bequest from the grandfather's estate, and is it permissible to retract the division after it has been agreed upon and each heir has received their share?

You have no share in your grandfather's inheritance because you are excluded by his son (your paternal uncle), for a son excludes a son's son from inheritance by consensus. What is known as the compulsory bequest law is religiously invalid and must not be adopted. Your grandfather's relinquishment of his share of your father's estate to you is considered a gift and is subject to the conditions of a gift. If it was not made during a death-sickness and you took possession of it during his lifetime, it is valid; otherwise, it is null and void. If it was made during a death-sickness, it is treated as a bequest and cannot be executed for an heir. In case of disagreement, one must refer to the Sharia court or to scholars.