I think it is important to distinguish between the temporal order of human history and the timeless order of the heavenly life.
From a historical perspective you are right that the resurrected and glorified Christ come after incarnation.
However, from the timeless perspective of God and those in heaven, incarnation, resurrection, and glorification, although distinct, they are seen at once and not as a temporal sequence of events coming one after the other.
When the apostle John was writing his first letter (1 John) Jesus had already resurrected, and John had seen him several times with his glorious body.
Accordingly, the passage “when Christ appears, we shall be like him, for we shall see him as he is” in 1 John 3:2, is clearly referring to the state we will have in heaven after the last judgement, and therefore it means that those who are in heaven are like the Son of God because they shall see Christ as he is, i.e.: “sitting down at the right hand of the Majesty in heaven” (Hebrews 1:3).
From this it follows that Melchizedek (who is like the Son of God, but is NOT the Son of God) necessarily shares in the state of someone who sees Christ “sitting down at the right hand of the Majesty in heaven” (Hebrews 1:3). As said, all this is perfectly fitting as it happens beyond space and time.
Notice that Jesus Christ himself refers to something similar when speaking about Abraham he claims:
“Your father Abraham rejoiced at the thought of seeing my day; he saw it and was glad.
‘You are not yet fifty years old,’ they said to him, ‘and you have seen Abraham!’
‘Very truly I tell you,’ Jesus answered, ‘before Abraham was born, I am!’ “
(John 8:56-58).
Here the Patriarch Abraham (in the unseen world beyond) is allowed to know and rejoice in the fact of the Incarnation. Nonetheless his vision is far inferior than that of Melchizedek, since Abraham is not spoken of as “being made like the Son of God”. And contrarily to Melchizedek, Abraham is not said to be “without end of life”: Abraham was not sinless, died, and was in the afterlife waiting for Christ’s resurrection. Melchizedek was sinless, did not die, and was in heaven enjoying the vision St. John describes in 1 John 3:2.
Finally, this interpretation is supported by the messianic Psalm 110 you yourself quote as well:
I would rather say that Hebrews 7 is developing the inspired theology of the messianic Psalm 110, on the basis of Jesus teaching in Matthew 22:42-45:
“What do you think about the Messiah? Whose son is he?” “The son of David,” they replied.
He said to them, “How is it then that David, speaking by the Spirit , calls him ‘Lord’? For he says,
‘The Lord said to my Lord: “Sit at my right hand until I put your enemies under your feet.” ’
If then David calls him ‘Lord,’ how can he be his son?”
Here Jesus states that David in Psalm 110 is speaking inspired by the Spirit, so that he (David) can hear what God says to the Messiah (Jesus) before the incarnation of Jesus, and this happens manifestly in heaven, beyond space and time).
In Psalm 110 we read as well:
“The Lord has sworn and will not change his mind: ‘You are a priest forever, in the order of Melchizedek.’ “
This is not a “metaphorical comparison” between two different priesthoods, but rather a solemn definition that the Messiah, when he comes, will take on the priesthood of Melchizedek and continue this priesthood forever.
This presupposes that the order of Melchizedek is a real priesthood at the moment David is speaking. But how is this possible, if Melchizedek is no longer on earth, and the only order in force at this time is the Levitical one? The author of Hebrews, undoubtedly “speaking by the Spirit” as well, interprets Psalm 110 as a declaration that Melchizedek is “like the son of God”, and so the order of Melchizedek really goes on in heaven: When the son of God became flesh, he became priest in the order of Melchizedek as well.
What “lasts forever” is Melchizedek himself as a human being with his body in heaven, and so lasts his order as well, till it is adopted by Jesus Christ. Hebrews 7:6-8 clearly establishes a strong contrast between the descendants of Levi, who were “people who die”, and Melchizedek, as someone who lived: The Levites were mortal priests and thus inferior to the High Priest Melchizedek, who did not die.