I’ve encountered a similar speculation that the manna in the desert was some kind of true Eucharist. Personally I lean against the idea, partially because of the worms that would corrupt the manna if an attempt was made to store it (meaning the person didn’t trust God to provide for them the next day). If it were the Eucharist this would be a terrible horror. Also there’s the manifest unworthiness of the people who ate it in general, and the large quantities eaten. I really think the manna was a type (as in image, model, prefigurement) of the Eucharist rather than the Eucharist itself.
Same with the bread and wine offered by Melchizedek. I’ve heard an explanation that the reference to him being without father or mother, birth or death refers to an early Rabbinic concept that if something isn’t recorded in the Bible it didn’t happen. This obviously isn’t literally true, but the author of Hebrews (likely a Jew writing for other Jews) seems to have taken the concept and applied a more figurative meaning to it, to emphasize the typological relationship between Melchizedek and Christ. Melchizedek doesn’t have any recorded father or mother, etc., prefiguring the eternity of the Logos, Who would become the true King of Peace and Priest of the Most High.
It seems quite plausible to me that the actual man Melchizedek was a Canaanite priest-king, one who followed the ancient, almost monotheistic cult of El (God) or Elyon (Most High) if to the people of Salem he was a different deity, a creator beyond even El, as he seems to have been in some Semitic cultures, or perhaps most likely a sort of combination of the two into a single Creator-King God, as the name given, “El Elyon,” suggests. He could therefore represent a good element of natural religion, before the Canaanites had reached the fullness of their iniquity (before which the Israelites would not be allowed to take possession of the Holy Land- hence the long delay). Clearly the Bible takes this “God Most High” to be the same as YHWH, and Melchizedek’s sacrifice of bread and wine comes to be seen as a prefigurement of the Eucharist. I don’t see why it would instead be Christ Himself going back in time and consecrating the Eucharist for Abraham or something like that, though I suppose it’s possible.