Is the Hajj valid for someone afflicted with obsessive-compulsive disorder (OCD) regarding the throwing of the pebbles (ramy al-jamarāt), circumambulation (tawaf), and the ritual walking (sa'i), and what results from that in terms of doubts about purity (taharah) and exiting the state of ihram, especially with the appearance of a yellow spot in the underwear after returning?

You are afflicted with Waswas (whispering doubts/obsessive thoughts). The cure for Waswas is to disregard and ignore them. Your Hajj is valid, and all your rituals are beyond reproach. Whatever you found on your clothes—if indeed you found anything—should be attributed to the most recent time it could have possibly occurred. So, cast away these Waswas and ask Allah for acceptance.