Mark 3
an abstract of the worlds view of someone who is mentally ill:
Positive Symptoms: The Psychotic Dimension
Religious delusions involve patients' belief that they have a special relationship with God normally reserved for bible or mythic figures, or with the spiritual world. Patients may state that they are the incarnation of the archangel Michael, for instance. They may complain of demon possession, of being able to speak to God directly and hear replies, or to be in communication with a spirit from another dimension. Patients may believe that they are God, or God's chosen messenger.
That would be taking my OP as if applying that all mentally ill patients are believers in Jesus Christ which some are not.
There are a wide range of afflictions for what society has deemed as mentally ill.
The question about Paul being "mentally ill" was in parenthesis "___" to refer to how society would lable Paul as such, but they would be wrong.
There is a spiritual warfare of which Paul would testify towards and thus fiery trials will come as we lean on Christ Jesus to get us through it.
Jesus spoke of tribulations that believers will go through.. even if it involves something continuous like a terminal illness that leads to death.
So that does not exempt Paul from being afflicted in a supernatural sense of hearing voices that accuses him as that would be befitting a messenger of Satan that buffets.
And buffetting Paul with accusations does not necessarily mean possession, but oppression.
Does not Satan whispers temptations in our ears? That does not mean he is in us.
So Paul being buffetted by a messenger of Satan and him asking God to remove this thorn in the flesh is just Paul's way of saying it was a personal affliction: not that he was possessed.
As for those that are "mentally ill" as per your quoted reference, Paul does not fall in that category of delusions of thinking himself God or highly special by having this thorn in the flesh when he acknowledges it as a humiliating affliction that would keep others from exlating him higher than they ought to think.
1 Corinthians 3:5 Who then is Paul, and who is Apollos, but ministers by whom ye believed, even as the Lord gave to every man? 6 I have planted, Apollos watered; but God gave the increase. 7 So then neither is he that planteth any thing, neither he that watereth; but God that giveth the increase.
Those mentally ill patients are working their delusions for others to exalt them higher than they ought to think: the opposite of what Paul says his thorn in the flesh was for.