Does the grandmother's death, after her refusal to leave the mother, indicate her displeasure, and what is the ruling on that for the mother?

Your mother was dutiful towards your grandmother, and her leaving her on the day of her death does not diminish her dutifulness, especially if it was out of obedience to her husband. And if she fell short once, that is a strong indication of Allah's forgiveness, for He says: "Your Lord is most knowing of what is within yourselves. If you should be righteous, then indeed He is ever Forgiving to the often returning [to Him]." And a person can show dutifulness to their parents after their death by supplicating for them, seeking forgiveness for them, giving charity on their behalf, maintaining ties of kinship through them, and honoring their friends.