Elijah does come first to RESTORE ALL things

The OT ended with Mal 4:

1 “For behold, the day is coming, burning like an oven, when all the arrogant and all evildoers will be stubble. The day that is coming shall set them ablaze, says the Lord of hosts, so that it will leave them neither root nor branch.

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It is a special day of the Lord. Label it D1.

5 “Behold, I will send you Elijah the prophet before the great and awesome day of the Lord comes. 6 And he will turn the hearts of fathers to their children and the hearts of children to their fathers, lest I come and strike the land with a decree of utter destruction.”

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Elijah would come before D1.

Four centuries later, the disciples asked Jesus in Mk 9:

11 “Why do the scribes say that first Elijah must come?”

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The scribes alluded to Malachi.

12 And he said to them, “Elijah does come first to restore all things.

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Berean Literal Bible:

And He was saying to them, "Elijah indeed, having come first, restores all things;

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Jesus supplemented Malachi's prophecy. Elijah referred to John the Baptizer.

Had John restored all things already?

No. The Greek word for "restores" (ἀποκαθιστάνει, G600) suggested a return to an original state or condition. It was a verb, present indicative active, suggesting an ongoing process. John, having come first before Jesus, started the process of restoration of all things, which will end on the last day. If John had restored all things already,

how is it written of the Son of Man that he should suffer many things and be treated with contempt?

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I.e., Jesus had to die on the cross as part of this restoration process.

The parallel account in Mt 17:

10 The disciples asked him, “Then why do the scribes say that first Elijah must come?” 11 He answered, “Elijah does come, and he will restore all things.

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future indicative active

12 But I tell you that Elijah has already come, and they did not recognize him, but did to him whatever they pleased. So also the Son of Man will certainly suffer at their hands.” 13 Then the disciples understood that he was speaking to them of John the Baptist.

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What was the great and awesome day of the Lord, D1?

It had two senses:

1. The immediate sense was the first coming of Jesus, along with John the Baptizer.
2. The future sense referred to the second coming of Jesus on the last day.

Had Elijah come to restore all things already?

No. Both Malachi and Jesus used prophetic language, which was open to interpretations. That's why Mark and Matthew's accounts used different tenses for the lemma G600.

This is my interpretation: John the Baptizer came to prepare the Jews for the way of Jesus, to start the process of the restoration of all things. Most importantly, Jesus' death and resurrection was the key part of this restoration process. One day in the future, on the last day, Jesus will return to complete this process.