Is one who memorizes the Qur’an then forgets it sinful? And is it obligatory not to memorize it for fear of forgetting it and thus being held accountable?

What a mother fears regarding forgetting the Quran is a whisper from Satan, and she should strive to memorize and review it. If she forgets something without any negligence on her part, then there is no blame on her, "for Allah does not burden a soul beyond its capacity."