It may also have had to do with Fr. Martin being assigned certain diplomatic missions regarding the church in persecuted areas like behind the Iron Curtain. Also, it's not really that uncommon for someone to request being taken out of the jurisdictional authority of a religious order or diocese where the superiors in question cannot be trusted to make prudent decisions. The case of Fr. Joseph Fessio, SJ, being just one example of the kinds of things that happen to priests who are considered too conservative or too orthodox by their superiors.
There is actually a scene in the film The Exorcist where the Jesuit character Fr. Karras, SJ mentions to another Jesuit that he thinks he needs a new assignment because he is having trouble dealing with the situations involved in the role of psychiatrist for the religious community. Not uncommon.
At the time that Malachi Martin left the Society of Jesus, I believe he would have had to have his writings approved by superiors in the order. That may have been a significant consideration. Another likely possibility is that the Pope may have wanted someone reliable that he could trust to observe and report back on trends in the U.S. That would be a completely valid reason and an honorable ecclesiastical assignment.
The reports of Malachi Martin having played some role in actual exorcisms in the U.S. would be another valid reason. And that also would explain the aura of confidentiality and secrecy which surrounds all cases of alleged diabolical possession and any official exorcisms performed. That he wrote a book on the subject is revealing.
Didn't know anything about Fr. Malachi until I read this piece, and now I'm fascinated. Plan on getting to the Library this week, and looking over some of his stuff.
"Another likely possibility is that the Pope may have wanted someone reliable that he could trust to observe and report back on trends in the U.S. That would be a completely valid reason and an honorable ecclesiastical assignment."
You are on the right track. Indeed he was to report back to the pope and to his successors on the throne. Hence his use of linguistic, literary, espionage, skills, and extensive network of contacts. An interesting side issue is that if one is to work and function in such a manner, one needs to travel both incognito, and at the same time cultivate acquaintaces on the "other side". If you catch my drift......
One should know your friends well.......but your enemies better!