So the assemblies have been given 'power and authority over all demons, and to cure diseases', or, to turn one over to Satan for the destruction of the flesh?
My 'method of interpretation' is perfectly in line with Charles Hodge's three simple rules:
1. The Scriptures are to be taken in the sense attached to them in the age and by the people to whom they were addressed.
2. Scripture cannot contradict Scripture.
3. The Scriptures are to be interpreted under the guidance of the Holy Spirit, which guidance is to be humbly and earnestly sought.
Me thinks you are boxing yourself into a corner. Almost all of scripture is written to a primary audience; either a person or group of people. Most scholars agree Matthew was written to a Jewish audience due to his genealogy in chapter 1 and his distinct Judaistic references throughout his gospel. Even though scripture has a primary audience it is binding upon, and applies to, our life. In chapter 18 Jesus was talking to His disciples. Almost every reputable scholar (Calvin, Henry, Gill, Rutherford, Spurgeon, Edwards, Dagg, Piper, MacArthur, Sproul et. al.), alive and dead, views Matthew 18:15-20 as establishing the procedure for church discipline. Even though the word ecclessia in verse 17 is translated "church" before the New Testament church was inaugurated, it applies to the church since God has always had one called out people.