I don't know for sure about Melchizedek. From what I found, many believe he was a regular human, and some believe he was a theophany. I even found the phrase you used about Hebrews "creating confusion". In any event, the main point is still that Christ did make appearances in the OT and the righteous did have faith in Him.
On theophanies in general, I found this excerpt from answers.com:
... The Encyclopedia Britannica similarly defines this as "a manifestation of deity in sensible form."[3] In the Judeo-Christian tradition, the Bible is the primary source of events which both Britannica and the New Catholic Encyclopedia cite as being theophanies.
The New Catholic Encyclopedia cites examples such as Gen 3:8a.[4] The same source then quotes Gen 16:7-14.[5] In this case, initially it is an angel which appears to Hagar, however it then says that God spoke directly to her, and that she saw God and lived (Gen 16:13). The next example the New Catholic Encyclopedia cites is Gen 22:11-15, which states explicitly that it was the angel of the Lord, rather than God Himself, speaking to Abraham (Gen 22:11a).[6] However, the angel addressing Abraham speaks the very words of God in the first person (Gen 22:12b). In both of the last two examples, although it is an angel present, the voice of the Lord Himself is spoken through the angel, and so this is a manifestation of Deity. The angel is therefore a preincarnate appearance of Jesus Christ.
There was no Christ prior to Incarnation. Christ means Messiah. The pre-incarnation Son of God is the eternal Logos. You can's speak of "Christ" before Incarnation. The righteous who believed theopany did not distinguish God from God the Word, or God the Spirit. The OT does not have those concepts. They had "seeds" (sporoi), just as other Abrahamic and monotheistic religions have. They did not believe in a Christian God. And it is a guess if they would have recognized Him after the Incarnation.
On theophanies in general, I found this excerpt from answers.com: ... The Encyclopedia Britannica similarly defines this as "a manifestation of deity in sensible form."
I have no clue what "sensible" means in this case. Does it mean as in "makes sense" or as something "detectable?" You get to see God without really seeing Him as He is. The appearance is only an illusion. His Incarnate presence was real. Big difference.
The Orthodox (and Catholics) celebrate the Theophany of the Lord following His Baptism, not as the OT appearances in "sensible" forms.