Does memorizing the Qur’an secretly from one’s mother constitute disobedience (ʿuqūq), or is that permissible? And is memorizing the Qur’an in three months while under academic pressure a sound decision Islamically?

Memorizing the Noble Quran is one of the greatest acts of devotion. The people of the Quran are the people of Allah and His elect. Your memorization of what is easy from the Quran, without neglecting your duties and without your mother's knowledge, is not disobedience. There is no contradiction between studying and memorizing the Quran if you manage your time effectively, and you should try to convince your mother. To memorize the Quran, reduce the amount memorized daily while repeating it, pay attention to revision, and ask Allah for help. It would be preferable if this were done under the supervision of a proficient female teacher.