This is in response to Derringer's comment.
First, Islam does recognize Christianity, the Quran says that Christians are the closest to Muslims and that they are with the Jews from the people of the book. The main difference is that Islam does not believe in the trinity and original sin doctrines and that Muslims believe that there was another prophet after Jesus.
You said that this demon from Iraq is from Islam. Demons are not from any religion since they rebel against God. Concerning moving from a country to another, they can travel on their own, flying in the air as I have read.
The priest conducting the exorcism was Father Merrill and he was assisted by Father Damian Karras who had doubts in his christian faith. He was chosen because he was also a psychiatrist. Apparently, they wanted to rule out that this possession was not a case of schizophrenia where people hear voices and/or see things that are not there.
Concerning the 360 degrees neck rotation, I agree with you that this anatomically impossible except for an owl which has more vertebrae (back bones) in its neck. The only explanation I could think of would be that the image of the demon was hiding the real person behind himself and it was only that image that was doing this apparent neck movement.
There was another movie (TV) in 2000 with Timothy Dalton called Possessed that is supposed to be more reliable as to what really did occur that inspired these 2 movies.