score:9
The crux of this question seems to lie in the final paragraph:
So, what specific priesthood did the line of Adam from Seth to Noah hold that would have to predate the other two priesthoods by at least four centuries?
The confusion leading to this question is cleared up in Doctrine and Covenants, section 107:
2 Why the first [order of Priesthood] is called the Melchizedek Priesthood is because Melchizedek was such a great high priest.
3 Before his day it was called the Holy Priesthood, after the Order of the Son of God.
4 But out of respect or reverence to the name of the Supreme Being, to avoid the too frequent repetition of his name, they, the church, in ancient days, called that priesthood after Melchizedek, or the Melchizedek Priesthood.
Therefore, the Priesthood from the time of Adam to Noah was the Melchizedek Priesthood; it just wasn't called that yet.