Does fasting become invalid if secretions from the teeth mix with saliva and the fasting person swallows them while asleep without being able to expel them while awake? And does fasting become invalid upon swallowing saliva if the fasting person suspects the presence of secretions along with a change in the taste of the mouth?

It is not permissible to swallow excretions other than saliva that come from the teeth, such as blood or food particles. Swallowing them intentionally is among the things that invalidate the fast. If the fasting person strives not to swallow them and something enters his stomach unintentionally, then there is no blame upon him, and similarly, if he doubts whether something reached his stomach. What enters the stomach from these excretions during sleep incurs no sin and does not invalidate the fast, based on the hadith: "The pen is lifted from three." If something precedes him to the stomach without his will, then his fast is valid. However, if the fasting person neglects to expel them and they remain in his mouth until drowsiness overcomes him, then the fast is invalidated according to many scholars, by analogy with one who puts water in his mouth without need and it precedes him. The preferred view is that there is no make-up required for the questioner, due to his not intentionally sleeping with them in his mouth. So his fast is valid as long as he exerts his utmost effort not to swallow these excretions, and it is safer to make up the fast if there is concern of negligence.