Actually Jambo's post above got me thinking that it could be the people getting the readings who were acting as the 'therapist role'.I like your hypothesis. I'd like to add that alternatively, if the critics of MPD are correct that MPD is an artificial condition, then the psychics could be like the therapists (rather than the patients), manipulating the subject's perceptions and creating the illusion that another presence in the room besides the psychic and subject.
After all they are the ones encouraging the presence of the other personalities.
It is they who the 'psychic' is trying to please by manifesting these other personalities.
It is they who the 'psychic' is trying to please by saying things that might be relevant or interesting to them.
It is their suggestions that may be helping to create the 'alter' of the dead person.
Another thing - does anyone know the ratio of female to male mediums and psychics? Because 85% of MPD patients are female. I wonder how closely this would correlate?