What is the ruling regarding a woman's refusal to memorize the Qur'an due to fear of fitnah (temptation/strife), despite the availability of Sharia-compliant safeguards that prevent khalwah (private seclusion) and ru'yah (unveiled viewing)? And is she sinful for that?

Memorizing the Quran is not an obligation for which a Muslim would be sinful for neglecting, except for Al-Fatiha. It is permissible for a woman to learn the Quran from a man when there is a need, provided there are legitimate conditions and regulations. There is no sin upon a woman for refusing to learn from a man, especially if she fears temptation; rather, her refusal might be obligatory upon her. She should strive to learn the Quran through women, or through recordings and the Mushaf.