Is my mother obliged to make up for the years of fasting that she missed out of ignorance, along with her regret, even if she does not know their exact number? And is it permissible for her to pay expiation (kaffarah) instead of making up for them due to diabetes, which makes fasting difficult for her, and what is the amount of this expiation?

Your mother must make up for the prayers and fasts she intentionally missed. If she cannot determine the exact number, she should strive and investigate to make up what she believes will clear her conscience. If she is ill and fasting is difficult for her, she may delay making up the fasts until she is able to do so without harm. If her illness is incurable, and she can make up the fasts during short days, she must do so, and she is not permitted to resort to expiation (fidya) as long as she is capable. However, if fasting permanently harms her, she should feed one mudd of food (approximately 750 grams) for each day. According to the majority of scholars, it is not permissible to give its monetary value. She also owes expiation for delaying the make-up fasts, which is feeding one poor person for each day, unless she was ignorant of the impermissibility of the delay.