Is a woman sinful for delaying the Maghrib and Isha prayers until midnight due to attending an event outside the home, knowing that she performed ghusl and prayed immediately upon her return, and that it was the last day of her menstrual period?

You should have checked for purity before the time for Maghrib prayer expired. It is obligatory for a menstruating woman to check for purity before sleeping at night and before every prayer. The obligation then narrows until there remains only enough time for ghusl (ritual bath) and prayer. Since you prayed Maghrib and Isha, you have fulfilled your obligation. You should seek Allah's forgiveness and be diligent in checking for purity when it becomes obligatory.