It is clear that after reviewing this wonderful outline of information, that the influence and existence of demons are prevalent in the world. The question though still stands, whether "a Christian" or a "Spirit-filled Christian" can be possessed by a demon(s). It is my contention, that in order to try to address this question we need to define what is a Christian, and what is a Spirit-filled Christian?
A Christian can be defined as an individual who believes in his/her heart, and confesses with his/her mouth, that Jesus Christ is Lord, and that he resurrected from the dead. Romans 10
A Spirit-Filled Christian can be defined as a Christian (above), who has been baptized (immersed) and filled with the Holy Spirit, and who now has the Spirit of Christ dwelling in him/her. Acts 1:5 (i added some additional scripture below)
It is hard to fathom that Christ would share his dominion with a demon spirit(s) if the believer has been filled with the spirit of Christ - and Christ has taken residence in his/her heart. For it is clear, that Satan himself was cast out of heaven when his great rebellion took place. It is important to understand that every Christian should desire to be filled with the Holy Spirit, in order to resolve some of the controversies regarding demon possession.
I do believe that Christians can be influenced, and at times assist in acts of rebellion against God's work - just ask Judas & Peter. But, considering that these acts were performed prior to the baptism of the Holy Spirit, I must contend that it is hard to fathom that the Holy Spirit would reside and make his home in the heart of a believer and share his residence with a demon spirit. Just take into account the word "unoccupied" in the scripture Matthew 12:43-45. The evil spirit found rest in the home, which was unoccupied, not necessarily in order, or cleaned up. Food for thought.
Here are just a few scriptures which is rich;
For John baptized with water, but in a few days you will be baptized with the Holy Spirit."
If you then, though you are evil, know how to give good gifts to your children, how much more will your Father in heaven give the Holy Spirit to those who ask him!"
And with that he breathed on them and said, "Receive the Holy Spirit.
But the Counselor, the Holy Spirit, whom the Father will send in my name, will teach you all things and will remind you of everything I have said to you
Peter replied, "Repent and be baptized, every one of you, in the name of Jesus Christ for the forgiveness of your sins. And you will receive the gift of the Holy Spirit.
When they arrived, they prayed for them that they might receive the Holy Spirit,
because the Holy Spirit had not yet come upon any of them; they had simply been baptized into the name of the Lord Jesus.
Then Peter and John placed their hands on them, and they received the Holy Spirit.
Then I remembered what the Lord had said: 'John baptized with water, but you will be baptized with the Holy Spirit.'
and asked them, "Did you receive the Holy Spirit when you believed?" They answered, "No, we have not even heard that there is a Holy Spirit."
And the list goes on...