Is it permissible for an elderly, immobile woman who uses diapers and prays from bed to fast Ramadan? And does her daughter's assistance with her ritual bathing affect her fasting?

If your mother is of sound mind, then fasting and prayer are obligatory upon her to the best of her ability. If she is able to fast, she should fast. If she is completely unable due to old age or illness, she should feed a poor person for each day. As for prayer, she should pray according to her condition. If she is unable to stand or sit, she should pray lying on her side, facing the Qiblah, and gesturing for bowing and prostration. She is obliged to perform ablution for prayer and remove impurities to the extent of her capacity. If impurity exits involuntarily, then hardship permits adopting the view of the Malikis who pardon it. She should perform everything she is able to do from the conditions and pillars of prayer, and what she is unable to do is waived for her. Your seeing her naked out of necessity does not harm or affect the validity of your fasting.