Are the thoughts that cross the mind of the questioner—that someone is listening to her during her prayer and supplication, which distracts her heart from devotion—considered waswasah (whisperings from Satan)? And are her prayer and supplication invalidated in this case?

This is a (waswas) whisper from Satan; you must strive against it and not give in to it. No harm will come to you if someone hears you, as long as your intention is sincerely for Allah. Concealing good deeds is better than revealing them, and more conducive to their acceptance, so strive to hide your acts of obedience as much as possible. Your previous deeds are valid, and what you mentioned does not affect them. Some deeds are legislated to be performed openly, such as congregational prayer. It is not legislated for a person to pray alone out of fear of showing off; rather, they should pray in congregation and strive to achieve sincerity.