Phizzy,
Both Melchizedek and Jethro were high priests of the monotheistic God that Jews worship today. They were also partial contributors or at least represent those peoples who contributed to the concept of "God" as recognized by the Jews.
Historically and according to anthropology, it appears that the Israelites adopted labels and some facets of God from the people living around the Fertile Crescent area and the areas referred to as Horeb or Mt. Sinai in Scripture.
Jewish theology and tradition agree that Abraham rejected the notion of the existence and worship of all deities, coming to see life and creation as the "effect" of a "Cause" greater than the human-made deity concept. In Abraham's mind it was this Cause and only this Cause that was responsible for the universe he could witness.
By likely attaching the monotheistic concept Abraham learned from his meeting with Melchizedek (whoever that was or represents) and Moses' meeting Jethro generations later, the Hebrews attached the words and labels these peoples used in their particular version of monotheism and we Jews attached it to what eventually we now call "God."
The God of Abraham that we Jews worship is therefore not a god in the traditional religious sense, but being greater than gods worshipped and invented by people we have attached some of the more common labels and words used for "deity" to this Cause. For exmple, the word ELOHIM in Hebrew is actually the word for a mighty wind storm or tempest. Since heathens often worshipped a storm god, Jews merely transferred the term to show that they attributed all such things to the First Cause of all that we worshipped.
Melchizedek is one of those clues in Scripture by which critical analytics can tie in with history and science to provide a clearer picture for understanding a more accurate evolution of the Jewish Revelation of God.