What is the theological flaw that might lead my mother, who shows extreme religious commitment, to curse the Divine Being, and will her repentance be accepted, and what is my role as her daughter in preventing this from recurring?

If a mother utters heinous words while not in her right mind due to extreme anger, she does not commit an act of disbelief (kufr) nor does she incur sin, because she is not held accountable. A word might be considered disbelief, but the speaker does not become a disbeliever if there is an impediment, such as extreme anger that impairs perception and will. Allah does not hold one accountable for words uttered without will and intention. Therefore, it is obligatory to show filial piety to the mother, treat her kindly, enjoin good upon her, and advise her to be wary of anger, as it is the key to evil, just as the Prophet, peace and blessings be upon him, advised: "Do not get angry."