The author uses Melchizedek to point his readers, and us, to the Lord Jesus. If we get caught up in the mystery of Melchizedek, we miss the point of the author - it's about Jesus and His superiority.
This is a difficult passage and yet, despite those difficulties, there are some things that are very clear: the clear warning to not fall away from Christ and the clear encouragement that we can trust our sovereign God.
The author encourages the Hebrews to mature to the solid food of the Word of righteousness. His passion is that no one would fall away - and part of that is becoming mature in the Word to discern good and evil.
Moses was faithful in that to which he'd been called. Jesus was (and is) faithful to that which He'd been called. But Jesus is greater than Moses - for several reasons.