Yes, there is a difference, and the difference is that theophany is Logos' manifestation without humanity. Was Melchizedek, not human?
An example of theophany is mentioned in Isa 9:6 (Septuagint)
For a child is born to us, and a son is given to us, whose government is upon his shoulder: and his name is called the Messenger1 of great counsel: for I will bring peace upon the princes, and health to him
1Greek : αγγελοσ (aggelos), angel
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.