Is dealing with fortune-tellers and soothsayers considered apostasy, and is it sufficient to perform ghusl and secretly utter the Two Testimonies to return to the fold, and what is the ruling on acts of worship performed during the period of dealing with them?

Visiting diviners and soothsayers is strictly forbidden. It is considered an act of disbelief (kufr) that expels one from the fold of Islam if it is accompanied by believing them and holding the conviction that they know the unseen. However, if the visit is made without belief in their claims, or with the belief that their knowledge comes from an external source (such as jinn), then it is not considered an act of disbelief that expels one from the fold of Islam. The non-acceptance of prayer for forty days for whoever visits and consults them means the loss of reward, not the invalidation of the prayer or the obligation to repeat it. As for repentance from major disbelief (kufr akbar), it does not require public declaration of the two testimonies of faith (Shahadatayn), but rather it is by acknowledging the truth and acting upon it.