[20a] Methuselah was the firstborn human son of the Archangel Michael

We know from John 5 that Jesus does everything that he sees his father doing...

19 Therefore, in answer, Jesus went on to say to them: Most truly I say to you, The Son cannot do a single thing of his own initiative, but only what he beholds the Father doing. For whatever things that One does, these things the Son also does in like manner (John 5). 

Then since God had a human son Adam, then so does Jesus! This son must be non adamic, a direct human son of Michael. Adam would have been the king of Eden had he not sinned. He did sin and Jesus fixed that sin by ransom and took all of Adam's rights. So Jesus holds the right to be the human king of the kingdom of God. He himself cannot be that king because he is now in a divine body and because we know he is the heavenly king of the kingdom, not its earthly king and because he has an angelic pre-existence so cannot have his home with mankind due to the divine equivalent of the prime directive of Star Trek.

6 And the angels that did not keep their original position but forsook their own proper dwelling place [it was OK to come down for some tourism, but not to live down here, to leave their proper angelic dwelling place, that violates the prime directive] he has reserved with eternal bonds under dense darkness for the judgment of the great day (Jude). 

Someone who has had a home in the heavens and knows all the angelic technology for software, hardware, biohardware, teleportation etc., etc., cannot come down and live on earth as a human for an extended period of time for fear that he might interfere with the human path to technological achievement by divulging angelic technological secrets to man. This is essentially the prime directive of Starfleet.

So he gives the human kingship to the one who has the legal right, his firstborn human son. But we know that Methuselah gets that kingship. So Methuselah is his firstborn human son. 

Furthermore Michael possessed Enoch, the father of Methuselah.

Michael obviously saw God sacrificing his firstborn son (himself) in order to validate the ARC, the Angelic Ransom Covenant - see intro28 and intro29. So following John 5:19, he too sacrifices his firstborn son, Methuselah, in order to validate the CRC, the Church Ransom Covenant - see intro28 and intro29.

So just as Jehovah's firstborn angelic son (by negotiation) was sent to die sacrificially to validate the ARC, the Angelic Ransom Covenant - see intro28 and intro29.

19 Therefore, in answer, Jesus went on to say to them: Most truly I say to you, The Son cannot do a single thing of his own initiative, but only what he beholds the Father doing. For whatever things that One does, these things the Son also does in like manner (John 5). 

So Michael is the firstborn angelic and human son of God (getting these rights from Satan and Adam). And Methuselah is the firstborn human son of Michael. But who is the firstborn angelic son of Michael? See intro20b.

Melchizedek

3 In being fatherless, motherless, without genealogy, having neither a beginning of days nor an end of life [He did have a beginning of days in the pre flood system as Jared. He did not have a beginning of days in the peri flood system as Melchizedek, since he beamed down as an adult into that system and therefore did not have childhood on his second or subsequent appearances. Melchizedek was resurrected into the post flood system just as the dead will be resurrected into the post lava flood system. He appeared as an adult rather than a baby and therefore missed out on the beginning of days part of his post flood life] but having been made like the Son of God, he remains a priest perpetually (Hebrews 7). 

So Melchizedek (Jared) was not a son of man. He was not a son of pre-fall Adam. He was a totally non adamic human son of Jesus. He had two surrogate parents, being the son of Mahalalel. He was gene zapped not to be a son of pre-fall Adam but rather to be a son of Michael. So from a human genetic standpoint he was both fatherless and motherless. So he was made like the son of God, who is Jesus. He was a gene zapped non adamic human

His beginning of life was his birth as Jared. But Melchizedek himself just beamed down as an adult and so had no beginning of days in the post flood system.