What is the ruling on going to a Christian infidel who treats only with the Quran, knowing that the questioner did not go to him to ask him, but for the Quran? And is that considered a sin, or disbelief, or permissible? And how does one repent if it is forbidden? And will her prayers not be accepted for forty days?

There is no harm in a disbeliever performing Ruqyah (definition) (exorcism/healing) for a Muslim, provided he performs it with the Book of Allah Almighty and the remembrance of Allah. This is not considered going to soothsayers. It is preferable for a Muslim to perform Ruqyah upon himself or to seek it from a Muslim reciter.