Can God forgive my grandmother—who used to go to soothsayers and believe their words, swear by other than God, and seek help from other than Him—even though she knew that shirk (associating partners with God) is unforgivable?

Your grandmother, and those like her who lived in a society ignorant of religious matters that are obscure to people like them, are excused. It is not permissible to declare specific individuals among them disbelievers, especially after their death. It is advisable to supplicate frequently and seek forgiveness for her, and to strive in inviting your relatives and those around you to good and teaching them matters of Tawhid (definition).