Is it permissible for a widow to work as a Quran teacher after the death of her husband, who had obligated her not to work, and is there any sin upon her or him in doing so, or would it lead to his displeasure?

There is no harm for a woman to engage in permissible work, especially if she is in need of it or if it fulfills a societal need, such as teaching the Quran, which is among the best of deeds. Obedience to the husband regarding leaving work is nullified by his death.