But he contrasted the teachings by the lake with the teachings to the disciples, not his teachings to everybody else. IT invalidates your point somewhat.
Mark 7:14 And having summoned the
crowd again, He was saying to them, “Listen to Me, all, and understand: 15There is nothing from outside the man entering into him which is able to defile him; but the things proceeding out of the man are the things defiling the man.”
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Mark 10:1 And from there having risen up, He comes into the region of Judea, and beyond the Jordan. And again
crowds come together to Him, and again, as He had been accustomed, He was teaching them.
2And the Pharisees, having approached, were demanding of Him if it is lawful for a husband to divorce a wife, testing Him.
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Mark 12:37 37David himself calls Him Lord. And from where is He his son?”
And the
great crowd was listening to Him gladly.
38And in His teaching He was saying, “Beware of the scribes, desiring to walk about in robes
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So in Mark, Jesus teaches the crowd in parables and not in parables. My point still stands: In 4:2 and 12:38, Mark states that he is quoting from somewhere, and Matthew 13 and 23 complete the picture.
Furthermore, if he highlights occasions where Jesus used non parable teachings, it's strange he never highlights occasions where Jesus taught the Sermon on the Mount, or the Lord's Prayer.
As we see from Mark 12:38, Mark can sum up a large section of Matthew in a few sentences. He does exactly that in 11:22-25, where he sums up the Lord's prayer.
24Because of this I say to you, all things whatever praying, you also ask, believe that you receive, and it will be to you.
25And when you may stand praying, if you have anything against anyone, forgive it, so that your Father in the heavens also might forgive you your trespasses.”
Matthew 6:11 Our daily bread, grant us today.
12And forgive us our debts,
as we also have forgiven our debtors.
You can wriggle as much as you like, but you can't get out of the fact that Mark never mentions either the Lord's Prayer or the Sermon on the Mount and yet these two preachings are central to Christian worship. The obvious explanation as to why he never mentions them is that he didn't know them. Therefore he wasn't copying Matthew.
Why is it so important to you that Matthew came first anyway? Would it make a jot of difference to your faith if it was the other way around?
Something I haven't yet mentioned is that Matthew is concerned to show, with multiple references, how Jesus fulfilled the Old Testament. The fact that the material in Mat. 5 contains quotes from the Mosaic law and is taught on a mountain is a strong hint that Jesus is fulfilling the Mosaic law, also given on a mountain. Even the false prophets Jesus mentions in 7:15 are mentioned in Deuteronomy, suggesting that for Mat. 5-7, Jesus is the fulfillment of Moses.
Mark on the other hand is writing for Gentiles and is not as concerned with the Mosaic law as Matthew, who wrote for Jews. Thus he omits much of the material in Matthew.