I still feel you are attempting to carve out a clever semantical argument here to block the Lord's work from being done rather than facing head-on the importance of deliverance prayer for many throughout the body of believers. Unlike you i have had many believers who were deeply suffering come to me for help and receive it. Whether they were technically "possessed" or technically "not possessed" but being tormented from the outside, or maybe their soul was being afflicted but not their spirit or whatever distinctions one might wish to make, it's really not very important where deliverance is concerned since either way, they are suffering and they need help. Whether they are possessed or "merely" afflicted, when nasty stuff is commanded to leave them in Jesus name, most of the time it does so instantly. So do you see yet how it's a senseless semantical filibuster to fixate on "whether a christian can be 'demon possessed'" and how this diversion just gets in the way of people who are in need from receiving help?
Among the believers i have helped, there are those who have been raped in childhood, abandoned, neglected or otherwise abused in their early formative years and they continue to be afflicted and tormented by this. There are those who are deep in depression, anxiety, fear, condemnation, anger, unforgiveness, confusion, inferiority, etc. There was an ex-satanic high priestess that was deeply tormented by many demon voices chattering continually in her head long after Lord Jesus called her unto Himself, she changed sides and came to Him. There are even those believers who report currently as believers being raped by demons. Yes these are people who proclaim Jesus as their Savior and Lord and call upon His name, but had not over many years of belief in Him encountered the freedom that He makes available. I know this doesn't fit into your model where everything is perfectly black and white and where everything works the same way 100% of the time since it seems a part of you deep within needs to see it this way, however false, misleading and truly detrimental this perception is. You can say this is not " demon possession" and that's fine, but i would hope at least you would have eyes to see that these believers are demon afflicted and in need of deliverance.
It would be wonderful if all people had to do was hear the gospel to be instantly delivered of every demonic stronghold in their lives... and for some rare people this actually does happen!..., but for many others, they have been sitting in church hearing the gospel message sometimes week after week for 30 years and remain deeply afflicted. What they needed all along was a body of believers who actually did the things Lord Jesus instructed His followers to do. Ideally this concept would be embraced and modeled by the church leaders, but alas, in the vast majority of cases within the church system, people like John MacArthur run the show and walking in the authority the Lord Jesus has given to those who truly follow Him is simply not what we see. Indeed, the blind are being led by the blind and the result is a weak and ineffective counterfeit system of religion that lacks the power we see in the early church.
A very important part of what i do to help people is to share the gospel with them including helping them to understand and experience God's love for them as never before... and another very important part is to walk in the authority to trample on snakes and scorpions that Lord Jesus has given to His modern followers - the very same authority He gave to His early followers - by commanding such things as depression, anxiety and fear to leave them in Jesus name.
The truth is that often believers are not in a place of faith to be able to do this for themselves and need the help of someone who has more faith and/or authority and/or gifting in this area. It's very similar to how some are able to heal the sick and others are not the same as was true 2000 years ago where "all do not have the same gifts and all do not have the same level of faith".
I know you hold staunchly to the false doctrine that all this ceased "when the bible was written" so that's where we part ways in our beliefs, but i have eyes to see the reality that my Lord's words are certainly and equally for those who would truly follow Him today. i experience this daily and so too can anybody else who would truly follow Him in every way.