I do agree that Melchizedek was not a messiah, but the rest of your post is very complex for something so simple. It is suited to a sermon, not a debate.
Melchizedek was a king who was also a high priest.
The song (Psalm 110) is about King David and shows that he was a king and considered a high priest. I feel the author of the book of Hebrews was trying to impress upon the Jewish audience that Jesus was a high priest in the order of Melchizedek, meaning he was also a king; not just a high priest as the Levites were (not in the order of Aaron).
"Peshat is what I say and derash is what you say." --Nehama Leibowitz