Is the fast of one who was forced to change the intention of an obligatory fast to a supererogatory one, or to break the fast upon hearing the adhan, then struggled and renewed the intention before the adhan finished, valid?

The cure for obsessive thoughts is to ignore them and pay them no mind. If you intend to fast a makeup day, do not pay attention to what Satan casts into your mind, such as the idea that you changed your intention. The same applies to prayer: as soon as you intend to pray a fard (obligatory) prayer, hasten to say the takbir and do not pay attention to the devil's whispers.