Complex sentence: Susan's father and many others believe that Steven has knowledge of Susan's fate and that circumstantial evidence points to him having played important roles in planning and assisting the disappearance and later covering it up by promoting implausible theories and cover stories to mislead authorities and media.
Split into simpler sentences: Susan's father and many others believe that Steven has knowledge of Susan's fate and that circumstantial evidence points to him having played important roles in planning and assisting the disappearance. Steven may have been assisting in a cover up by promoting implausible theories and cover stories to mislead authorities and media.