Is my quick rising from prostration in congregational prayer, out of fear that my mother might be surprised by my prolonged prostration, considered hypocrisy that invalidates the prayer?

Your prayer is valid as long as you have achieved ṭuma’nīnah (tranquility/stillness). The follower should not prolong prostration or bowing after the Imam, for the Sunnah is to move immediately after the Imam, as the Prophet, peace and blessings be upon him, said: "When he says Allahu Akbar, then say Allahu Akbar." If you abandon prolonging prostration or other acts of obedience for fear of showing off (riyāʾ), then this itself is riyāʾ. You must be wary of the whispers of Satan, who may tempt you to abandon good deeds for fear of riyāʾ. Proceed with your worship, mindful of Allah's gaze upon you, and pay no heed to the gaze of creation.