Must one make up for the missed fasts of days on which the obligatory prayers were not performed, or is it sufficient to make up for the prayers only, and is a specific expiation due?

The cure for whisperings (waswas) is to ignore them. Your fast is valid even if you missed some prayers, but you must repent and make up the missed prayers. You are not required to make up anything more than the days you missed fasting. As for continuous flatulence (sils al-reeh), if it reaches a point where you cannot purify yourself and pray within the prescribed time, then perform ablution (wudu) after the time for the prayer has begun and pray as much as you wish. It is also permissible for you to combine two prayers.