I agree with all you said if the incestrous man is a believer. But can we assume they were all believers? Paul definitely creates a blur here by grouping everyone in church as brethren / saved. Imagine we did that at our churches
.
Has Paul got a point we are missing? Perhaps all who come under the teaching of Jesus regularly have to be saved?
I /we all discern brethren in church as sheep + wolves (can be possessed) + sheep acting as wolves (incestrous man). But Paul distingushes between sheep + sheep acting as wolves. It is interesting.
We could say, without any doubt at all, that if a person is in the Church then they are saved. But, as you point out here, mixture has come in. If we look around us today, we can see that mixture - the sphere of Christendom, which contains everyone who
professes to be Christian, real and unreal, and then within that sphere the Church, the body of Christ. The wheat and the tares are growing together at the moment in the same field.
So, wolves have come in, "false brethren brought in surreptitiously, who came in surreptitiously to spy out our liberty which we have in Christ Jesus, that they might bring us into bondage" (Galatians 2:4), "spots in your love-feasts, feasting together with you without fear, pasturing themselves..." (Jude 1:12). They're here, they would try to get in amongst us, but we're not to mix with them - they're to be put out. We have to withdraw from their iniquity (2 Timothy 2:19). These are unconverted people, not part of the Church, but interlopers.
You're absolutely right that Paul distinguishes between the wolves and, we might say, the straying sheep. We get that in verse 9-13.
"I have written to you in the epistle not to mix with fornicators; not altogether with the fornicators of this world, or with the avaricious and the rapacious, or idolaters, since then ye should go out of the world." (1 Corinthians 5:9-10). These are the unbelievers, the worldly people, people "of this world". We can't keep company with them, because we should go out of the world.
"But now I have written to you, if any one called brother be fornicator, or avaricious, or idolator, or abusive, or a drunkard, or rapacious, not to mix with him; with such a one not even to eat." (1 Corinthians 5:11). One called a brother would be a believer, and as such he is held to a higher standard - there are further things introduced here which would be grounds for his exclusion from fellowship.
"For what have I to do with judging those outside also? ye, do not ye judge them that are within? But those without God judges. Remove the wicked person from amongst yourselves." (1 Corinthians 5:12-13). When a brother is caught up in these things, he must be removed, he has to be put under assembly discipline. We are responsible to guard the fellowship, to judge them that are within, and to remove wickedness from among us. Unbelievers, worldly people, Paul does not judge - their conduct is between them and God. But when someone is saved and comes into fellowship, then they are responsible to keep themselves from wickedness. If they don't, they can expect that their brethren must take action to remove wickedness from their midst - it cannot be tolerated. This is a substantial subject in itself, of course, one which we could go into a lot deeper. But I feel it's important to emphasise that love governs the dealings with a brother, no matter what he's done or is doing. Love and faithfulness go on together, and if we love the Lord and are acting in faithfulness to Him, we have to remove the wicked person. But if that person is a believer, we love them. It's not a matter of kicking them out, shutting the door and having done with the matter. It's an intensely sorrowful matter if a brother (or sister) gets into this state and has to be excluded from fellowship. We see that in the type of leprosy in the Old Testament - corruption in the flesh, the activity of man's will. The leper had to be excluded from the camp of Israel, but God gives detailed instructions with a view to their restoration. We should always have restoration in view, because we love the person, and we want to see them restored to their place. Moreover, the Lord loves the person, and it grieves His heart when any believer has to be excluded from practical fellowship because of wickedness - we should do what we can to restore them, as quickly as possible. In Leviticus where we get instructions about leprosy, it's the priest who cleanses the leper. We need brethren to have a priestly spirit, to have spiritual power to restore someone who's got away into a sinful course.
Everyone is knocking you for saying ''us'' with possession. But I believe there is such a thing as a carnal Christian who can be possessed. 
