Luke 3:10-14 contains exchanges between John and three groups of people (the crowd, tax collectors, soldiers) that is without parallel in the other gospels... John's preaching includes imperatives to share clothing and food with those who have none (ie. the destitute) and for the tax collectors and soldiers not to overtax or extort money respectively, to be content with their wages. He also relocates to 3:19 reference to John's condemnation of Herod for his marriage to Herodias from where it appears later on in Mark and Matthew... the particular problem is that she divorced her first husband who was Herod's brother. Adulterers and those who oppress workers, widows and orphans (these latter two the most destitute groups), with specific mention of wages, are among those who the Lord who comes will judge (Mal 3:5) --- that Luke crafts John's preaching in such terms as he does is quite a coincidence if he is not doing so with an eye on the text of Malachi and making sure those thee singled out for judgment are presented with a call to repentance (Luke 3:7-9) followed by a model for ethical living.
What if Luke crafted John's preaching according to what John actually preached? Since he wasn't there personally perhaps he had heard first hand stories of JB and what he preached as he traveled with Paul. It sounds like your are saying that Luke (who was possibly a gentile) cherry-picked from some of what he had heard that John had taught to mimic the text of Malachi. If Luke was a gentile wouldn't he have to have had knowledge of the writings of the Jewish prophets? Would that be a stretch for a gentile to have such knowledge during that time?
I've pointed out there is no evidence at those points in his narrative that Luke appeals to the Malachi text that he links Jesus to the "messenger of the covenant" --- this is all I can or need to offer in defense of my position. It is actually incumbent on you, as the one making the positive claim concerning this equation, to provide the evidence of such... for example, similar to what I do above for the positive claim I made about Luke's reliance on Mal 3:5 for the unique content of John's preaching he provides. Of course, if there were any evidence supporting the idea that Luke (or any of the other gospel writers) viewed Jesus as "the messenger of the covenant" in Mal 3:1, I would already have acknowledged it and be arguing that such is the position of Luke or other early Christian authors...
But Luke does have the quote from Isaiah and if Luke was an intimately related to the text of Malachi as you suggest then I think it is reasonable that Luke could put the similar texts of Malachi 3 and Isaiah 40 together to draw other corollaries and conclude that Jesus is, indeed, a messenger of YHWH.
The first would be that a voice/messenger who is preparing the way of the LORD.
The second would be that in each text a messenger is preparing the way for YHWH to come and that the people will see YHWH.
The third would not be a corollary but a difference in punctuation between the NSRV and the NET bible. The NSRV put a period between the
Lord coming to his temple and
the messenger of the covenant in whom you delight but the NET bible did not put the punctuated it differently. I think without the period, the passage is easily understood that
the LORD whom you seek 1.
will appear in his temple and 2.
is the messenger of covenant in whom they delight.
John the baptist isn't the messenger of the covenant neither is he someone in whom the nation of Israel had their hopes pinned on. They are anticipating the Messiah to show up just as was Herod around the time Jesus and John were both born.
The LORD is the one who is the messenger of the covenant and the one the nation is longing for. Luke 3:15
Isa 40:3 A voice cries out: "
In the wilderness prepare the way of the Lord; make straight in the desert a highway for our God.
4 Every valley shall be lifted up, and every mountain and hill be made low; the uneven ground shall become level, and the rough places a plain.
5
Then the glory of the Lord shall be revealed, and all flesh shall see it together, for the mouth of the Lord has spoken.”
Mal 3:1 See, I am sending
my messenger to prepare the way before me, and
the Lord whom you seek will suddenly come to his temple. The messenger of the covenant in whom you delight -- indeed, he is coming, says the LORD of Hosts. NRSV
Mal 3:1 I am about to
send my messenger, who will clear the way before me. Indeed,
the Lord you are seeking will suddenly come to his temple, and the messenger of the covenant, whom you long for, is certainly coming,” says the Lord of Heaven’s Armies. NET
Jesus was a messenger/prophet of his Father's words. He only spoke what his Father told him to speak regarding the kingdom and the good news and other aspects of his messages to the crowds and the leaders.
John 3:34 34
He whom God has sent speaks the words of God, for he gives the Spirit without measure.
John 7:16-17 16 Then Jesus answered them, “
My teaching is not mine but his who sent me. 17 Anyone who resolves to do the will of God will know whether the teaching is from God or whether I am speaking on my own.
John 8:28 So Jesus said, “When you have lifted up the Son of Man, then you will realize that I am he and that I do nothing on my own,
but I speak these things as the Father instructed me.
John 12:49-50
for I have not spoken on my own, but the Father who sent me has himself given me a commandment about what to say and what to speak. And I know that his commandment is eternal life.
What I speak, therefore, I speak just as the Father has told me.”
John 14:10 Do you not believe that I am in the Father and the Father is in me?
The words that I say to you I do not speak on my own, but the Father who dwells in me does his works.
Lastly the disciples see the Lord Jesus in just about everything in the OT. The entire OT is about Jesus Christ to them. They would surely understand Malachi 3:1 as I do.
In Luke 1 both Mary and Zechariah mention the promises that God made to their ancestors and are now announcing its fulfillment in the baby that will be born. Moses spoke of a promise YHWH made that the Lord would raise up a prophet like himself. Deut 18:15-18
Jesus was that prophet and just like Moses, Jesus would be involved in forming a covenant with the nation of Israel. The disciples knew all of these things and understood them as fulfilled in Jesus. And later in Luke 20 Jesus states that the glass of wine they drink is symbolically "... the
new covenant in my blood."