What is the ruling on fasting the days of doubt for making up for Ramadan, and is fasting obligatory when one predominantly assumes non-observance, and what is the ruling on one who doubts a vow to abandon a sin, and is expiation for the vow incumbent upon him?

If you doubt whether you need to make up for one or two days, you are only required to make up for one day. The second day, which is subject to doubt, is not obligatory, as mentioned by Sheikh Ibn Uthaymeen. As for doubt concerning a vow, it holds no weight, and expiation is not required. It is advised to ignore obsessive whispers (waswas) and doubts and to turn away from them. Therefore, do not fast a day if you doubt its obligation, and do not offer expiation if you doubt its necessity.