What are the boundaries of the mouth when fasting, and what is the ruling on doubting whether some saliva has reached the lips?

Fasting is not invalidated by doubt, especially for one who suffers from excessive scrupulosity (waswas), and it is obligatory to disregard the whispers of Satan. Swallowing saliva does not invalidate fasting, and there is no need for a make-up fast unless the fasting person swallows something other than their saliva. Traces of spittle on the lips do not invalidate fasting due to the difficulty of guarding against them.