Is the sensitivity of my client to experiencing cannabis-induced psychosis just an innate part of his genetic makeup, or because of a susceptibility caused by the prior long-standing possession by spirit meddlers having altered his brain function?

It is actually both because both components are important and determining whether he is a person susceptible to develop this difficulty or not. One or another by itself would not cause as severe a problem as he exhibits. It is the two factors working together in the worst possible way that bring about the severity of the consequences, making him truly lose control of himself and thus put himself at risk of the disordered thinking that comes about when he is high.