Is a sleeping person held accountable for what he experiences in a wet dream, and why does one feel accountable to God for this, despite the pen being lifted from the sleeping person?

The matter is as mentioned: the sleeping person is not held accountable for their actions because they lack intention and free will. The self-accountability that occurs during sleep does not mean it is by choice; rather, it is a manifestation of inner thoughts. It is advisable not to be preoccupied with visions and dreams and not to base anything upon them. Ibn Muflih narrated from Ibn Sirin that he used to say to those who asked him about visions: "Fear Allah and do good while awake, for what you see in your sleep will not harm you."