Am I obligated to offer an expiation for an oath (kaffarat yamin) or to fast if I swore not to make a vow, but then made one; or if I vowed to fast or pray in order to get rid of the whispers of vowing; or if I stipulated prayer with a specific surah, knowing that I suffer from obsessive-compulsive whispers regarding prayer, purity, and ablution?

The cure for obsessive thoughts (waswas) is to ignore them and pay them no heed. Any vow or oath made under the influence of waswas is not binding, nor is it binding if you broke an oath due to forgetfulness or under the influence of waswas, because a person afflicted by waswas is, in this situation, considered to be under duress. A vow is only binding if it is expressed in words that indicate commitment. If you intended to recite a specific surah in prayer but did not do so, there is no harm, and your prayer is valid as long as you have fulfilled the conditions and pillars of prayer. Repel obsessive thoughts concerning purity and prayer and do not pay attention to them, for yielding to them leads to great evil.