Does my mother no longer have to make up the prayers she missed after she committed to prayer and performed 'Umrah three times? What must she do to make up for what she missed, and should she perform supererogatory prayers?

Prayer holds a great status in Islam; it is the first thing a person will be held accountable for. Whoever observes it will succeed, and whoever neglects it will fail and lose, as indicated by the severe warning in the Holy Quran. One who abandons prayer, denying its obligation, is, by consensus, a disbeliever. However, there is a difference of opinion regarding one who abandons it out of laziness. Your mother must make up all the prayers she missed since puberty. If she cannot ascertain the exact number, she should make up what she believes will clear her conscience. It is permissible for her to follow the opinion of Sheikh al-Islam Ibn Taymiyyah, who stated that there is no need to make up prayers missed without an excuse. However, the majority opinion is closer to piety and caution in religious matters.