What is the ruling on places and clothes that have been mixed with secretions that their owner presumed to be impure due to his waswasah (obsessive whispering), and what is the ruling on his prayers that he performed after imagining the emission of wind or drops?

Your previous prayers are valid, and the places where you sit are pure and do not become impure by your sitting on them with wet clothes. In fact, your wet clothes are pure and are not judged to be impure merely based on doubt about something having exited. You must turn away from obsessive thoughts, as they are from Satan.