Jesus delivers the first 14 verses with his hands on the shoulders of a child. He is an example of humility necessary for everyone who would enter the kingdom. He is cause for a warning to any who might lead a little one astray. Is "his angel" a reference to a guardian angel?
The procedure of Matthew 18:15-18 is for interpersonal offenses. I can see where other offenses that may need to be dealt with other ways.
The kingdom of heaven is like this servant who has been forgiven much and who then struggles to forgive a fellow servant. Forgiveness comprises the whole fabric of being a Christian. Jesus connects my having been forgiven much with the need for me to forgive much. This is one of the fundamental structures of being Christian.