Well - I’ll admit it’s a pretty good argument, but I’m afraid it is unconvincing.
That’s okay, not trying to convince anyone.
I like that you desire to set the passage in the larger text, but your interprative conclusion is based on the assumption that Jesus is speaking first to the way things were currently done, (Mt 18:15-18) and THEN explaining how it should be done (Mt 18:19-20).
In support of this you offer the fact that Jesus would often say - “You heard it said…But I say…”,
Well, it goes far beyond that one construction, that’s just representative of thew theme of the whole book. It’s really a brilliant piece of writing, IMO. Maththew starts out with Jesus challenging the cultural understandings, with the Beatitudes. The culture of the Jews was that if you sufer misfortune, it is because of sin. You are cursed, IOW. But Jesus says, "Blessed are the poor in spirit, those who morn, the meek. He says the lowly are Blessed, not cursed. Then He telsl people what they need to do, hunger for rif ghteousness, be merciful, be peacemakers. And He warns that some wil be persecuted for the sake of righteousness.
And he is not in the Temple from which all the teachings flow, He is not speaking to those in the front, inside the walls, the rich and powerful. He’s out on a hillside speaking to everyone. It starts out, challenging the whole system by which that world operated and the basic system of belief. He then goes on to challenge the more specific but churchwide teachings in the “you have heard it said…” part. It’s like a poem, with great rhythm and beauty. It would have been sung when proclaimed. Matthew also abandons that form, he shifts the pattern, but the same things is going on: Jesus replacing what is in error with what is correct. In Chapter 6 He teaches about giving alms secretly and praying secretly and points out how it is done in an unrighteous way by hypocrites. So He has moved to something more specific, a class of sinner.
He replaces their wrong prayer with “right” prayer: the Our father.
having established the pattern, possibly to save time or change pace, Matthew then gives i us what is a “sayings Gospel:” in a set of swift teaching references throughout Chapter 8.
And the listener has to ask: “By what Authority can He do these things?” And so the miracles start, so that we know he is Divinely empowered. This is made perfectly clear when Matthew finally, after establishing all of this, in Chapter 9 shows Jesus meeting the challenge of the church in the form of scribes, accusing Him of blasphemy. And Jesus, in 9:6, claims His authority.
And then HE CALLS A TAX COLLECTOR!! Yowzer, how outrageous is that? Now there are more miracles to cement the idea of His Divine mandate, but the Pharisees voice doubt, so we have moved right up the ladder from scribe to PTB. The writer foreshadows what is to come.
We get an intermission of sorts from the tension and drama while Matthew gives the solid basics of Jesus’ teaching, again warns of persecutions, sends out representatives to spread His words, and assures people of His Grace and support. . Then, Chapter 12. Jesus has so far challenged the culture, the specific practices, the forms of sin, but now, He will confront individuals who are the Church of His time. He says He is greater than the Temple, He challenges somethings as basic as no work on the Sabbath. When the PTB says He is working for the Liar, He directly answers them and implies* they* are the ones blaspheming. He identifies them as “a brood of vipers” (nesting in the caves, they prevent the use of tombs, which are the gates to the afterlife, which they cannot use, but they prevent others from entering) - he is again, challenging the prevailing belief or authority and replacing it with His own Truth.
Then He does something really radical by replacing his own family, with the family of humanity. This is the literary form and theme of the book, a newer covenant, New Testament. The writer makes it obvious from the beginning, but the theme carries through, woven throughout until the very end so the reader can accept the glory of the Cross, instead of what all believed: it was the most shameful death and the person was lost. And so be prepared for the most outrageous unbelievable thing of all: that Jesus is alive and well and resurrected.
So the fact the in this specific passage, Jesus doesn’t use the same words He used on the Mount, isn’t any indication He isn’t doing the same thing He has done all through the book, where He also didn’t use those specific words, but still replaced the old ways of doing and thinking with His Way by His (God’s) Authority. (con’t)