None of your scriptures demonstrate that Christians can't have demons.
And the very fact that we are commanded to cast them out refutes your false theology.
If salvation stops demons, why did Jesus command us to cast them out?
The distinction between possession and oppression must be made in order to understand: --A demonically possessed person is without control of themselves to the degree of possession; that is, a person possessed by a few demons, or by less powerful demons, retains much of their autonomy, yet a person possessed by many demons, or by powerful demons, retains almost no autonomy.
Demonic possession equates to actual loss of physical control, but only to the degree of possession.
Demonic Oppression, however, is without loss of actual autonomy, because it is an external attempt by demonic forces to influence the person to commit to do things that will open the door to possession.
Christians are to resist these forces, and they will flee from Christians; however, none-Christians are without power to resist.
We are commanded to cast demons out, and to then teach the gospel, and baptize believers for the gift of the Holy Spirit, Who will then occupy the now empty house, and begin His work in that person.
The Holy Spirit, unlike demonic spirits, will not violate a person's God-given autonomy, but will instead, become one with that person, and start to conform them into the image of Christ.
Christians, are by definition, like Christ; and like Christ, they are free from demons.
Oppression and Possession are not synonymous.
It is absurd to think that the Holy Spirit will dwell in the same place a demon does--and even more absurd to think that a demon can occupy the dwelling place of the Holy Spirit.
Open your eyes, and turn from this doctrine, and accept the truth, that where the Spirit of the Lord is, there is liberty!