Welcome to the chat rooms Katia. Michael is just that, an angel and an arch angel at that. He is not God, and he is not the pre-incarnate Jesus Christ who is known in the Old Testament as "the angel of the Lord." Also, Melchizedek is not Jesus Christ as some believe. This is nicely explained in the following article.
https://insight.org/resources/article-library/individual/melchizedek-as-a-type-of-christ
The reason that it is impossible for Michael the arc angel cannot be the angel of the Lord in the OT is based on infallible proof (in my opinion) at Genesis 22:11-18. At vs15, "Then the angel of the Lord called to Abraham a second time from heaven, vs16, AND SAID, "By Myself I have sworn declares the Lord, because you have done this thing and have not withheld your son, your only son, vs17, indeed I will greatly bless you and will greatly multiply your seed as the stars of the heavens, and as the sand which is on the seashore; and your seed shall possess the gate of their enemies."
Here's the interesting part taken from the book of Hebrews in the New Testament which refers to the above. Hebrews 6:13-16, "For when God made the promise to Abraham, since He could not swear by no one greater, HE SWORE BY HIMSELF, vs14, saying, I will surely bless you, and I will surely multiply you." In other words, Michael is not Jesus Christ or should I say the angel of the Lord in the OT because angels cannot swear oaths on behalf of God Himself. Does this make sense?
IN GOD THE SON,
bluto