Is it permissible to boycott one's grandparents after an attempt at enjoining good and forbidding evil that was misunderstood, leading to feelings of resentment and hatred on their part, and my own emotional distress from their accusations?

Enjoining good and forbidding evil is an obligation upon Muslims. This includes the father and grandfather, while being careful to maintain etiquette and gentleness in speech. If your intention is legitimate in changing the evil, then there is no sin upon you for the anger of your grandparents. However, do not respond to this with abandonment. Rather, explain your intention to them, apologize to them, seek help from someone they approve of, and be patient with their harm, seeking reward from Allah.