The things that you describe are strange to me. I have no experience with demonic possession at all, but I grew up believing in it. I was raised Catholic, and the subject of demonic exorcism was talked about from time to time. It always inspired fear in me, but since embracing the teachings of The Urantia Book, I have no fear from such things. My experience with God has proven to me that God is stronger than any evil...and I am glad to read that you are also unafraid of what you saw.
You are right in thinking that these evil forces cannot invade the human mind unless their presence is invited, but I have no way of judging whether such an afflicted person as you describe has invited evil forces, or whether they have a mental weakness that somehow provides an avenue through which evil forces can work on the physical mechanism without the mental consent of the afflicted. Strange things do happen in this world, and I cannot explain with certainty how something like this happens.
The Urantia Book teaches us:
77:7.7 Even prior to Pentecost no rebel spirit could dominate a normal human mind, and since that day even the weak minds of inferior mortals are free from such possibilities. The supposed casting out of devils since the arrival of the Spirit of Truth has been a matter of confounding a belief in demoniacal possession with hysteria, insanity, and feeble-mindedness...
The entire group of rebel midwayers is at present held prisoner by order of the Most Highs of Edentia. No more do they roam this world on mischief bent. Regardless of the presence of the Thought Adjusters, the pouring out of the Spirit of Truth upon all flesh forever made it impossible for disloyal spirits of any sort or description ever again to invade even the most feeble of human minds. Since the day of Pentecost there never again can be such a thing as demoniacal possession.
So, we have the assurance that even in the weak person, demonic possession of the mind cannot happen.
There is a class of "subnormal" humans who do not have healthy mental functions:
113:1.3 The subnormal minded—those who do not exercise normal will power; those who do not make average decisions. This class embraces those who cannot comprehend God; they lack capacity for the intelligent worship of Deity. The subnormal beings of Urantia have a corps of seraphim, one company, with one battalion of cherubim, assigned to minister to them and to witness that justice and mercy are extended to them in the life struggles of the sphere.
Again, not having seen what you've seen, I can't judge, but one wonders whether such a things as you describe might have happened in such a "subnormal minded" individual - one who has not got the capacity for normal will function. For all I know, this could be an elaborate hoax perpetrated upon such an unfortunate being meant to make someone else appear more powerful than they really are when they are able to "exorcise" this supposed demon. I confess that I have no easy answer for this, as I have never seen anything like what you are describing, except in the movies.
Knowing that even the subnormal mind is cared for by an angelic corps, it is up to those of us who trust in the dominance of good to pray sincerely for the protection and deliverance of all such unfortunate persons through the angels' ministry. We have to remember that the forces of evil are real, but even though they may be real, they are not equal in either force or reality to God, or Goodness.
Thanks again for your note; I wish I could explain this situation away easily, but sometimes, we just have to trust in the goodness of God, pray for those who are afflicted in body or mind, and be assured that in God's world, all things really do work together for good eventually.