To that point, was the man in the region of the Garasenes someone who had entered into that sort of agreement? We're not told, but it certainly doesn't seem he was a willing victim. And neither were the pigs.
I suggest you look into People of the Lie by Scott Peck as a starting point for this subject (human and demonic evil). I think you will find you're are taking an unnecessarily narrow view of this matter.
Additionally, many things are invisible and lack direct scriptural support or linkage, but are still facts. BTW, not all of the people who assist in exorcism are believers. Often, the medical and psychiatric staff are atheists (or at least not devout believers), yet you are saying their direct experience has no value to someone studying this subject.
I think I am beginning to put little stock in your opinion.