Hebrews 5:7 says what it says. It says Jesus prayed to the one who could save him from death which is exactly the same sort of thing you and I would do because we aren't God. Jesus is a man Mattie.
The order of Melchizedek is a play on a technicality. Melchizedek isn't alive and immortal walking the earth right now just because his death record was lost. He died of course, but since there is no genealogy technically proving it then it can't be technically proven that the order of Melchizedek ever ended. The resurrection of Jesus is exploiting a loophole in this logic under the idea that Jesus is alive forever now. In a sense, he has something in common with Melchizedek and is, therefore, a priest forever. On that matter of Jesus being a priest, even right now in heaven, is proof that he is not himself God, but rather is a mediator and intercessor between God and the people just like all of the other priests before him.
Yes I agree that the sacrifice of Jesus was that of a sinless man, but it isn't the skin and bones of someone that compose what constitutes the sinless man, but rather the soul of said man. This is why it is the soul who sins who dies (Ezekiel 18:19,20) rather than flesh itself that can sin because sin is ultimately born of desire and what comes out of the heart (not the organ, but the actual thoughts and desires) of a person.
So who was sacrificed was the soul of Jesus actually. On that point, the soul of God was not sacrificed. In orthodox trinitarianism the essence between the members cannot be divided or else the whole system trainwrecks. If Jesus was God then, in effect, God died even though He is immortal. Believe it or not (I guess you won't believe it) Jesus was a sinless man who was such because God is his Father and he learned his behaviors and mannerisms from God (his Father.) Therefore he never learned how to sin, but rather how to be completely holy and righteous. He also taught people that ceasing their sin activity completely is attainable in the here and now.
Thank you for providing a powerful argument against the deity of Jesus.
Isaiah 53 (KJV)
10Yet it pleased the LORD to bruise him; he hath put
him to grief: when
thou shalt make his soul an offering for sin, he shall see
his seed, he shall prolong
his days, and the pleasure of the LORD shall prosper in his hand.