Christ's death was all entirely part of the same covenant. You are taking this passage past what it is saying. First of all there was provision for the forgiveness of sins in the Law of Moses. (Leviticus chapter 4,5 & 19; Numbers chapter 15; Psalms 32; and Jesus forgave sin all over the place prior to the cross).
All forgiveness is through Christ, no matter what dispensation you're talking about. Before the fist century God looked forward to the cross and now He looks back to it. This would be the case with or without the Body of Christ (i.e. the Dispensation of Grace). what could be. Thus it has never been your works that save you, its has ALWAYS been the blood of propitiation, the blood of animals only being a symbol and shadow of that which was to come, namely Jesus, God's only perfect lamb. Everything in the whole Old Testament was about Christ. From the clothing God fashioned for Adam to Abraham's sacrifice of Isaac to Moses striking the rock (the first time) to etc, etc, etc.
There's plenty more that could be said here. For instance, John the Baptist baptized people "for the remission of sins" (Mark 1:4) but if that isn't enough to refute your position here I'm not sure what could.
Of course! No one is suggesting that where no changes for Kingdom believers. The law clearly changed because now, after Calvary, all of the sacrificial rituals and related aspects of the law had been fulfilled and thus no longer had any effect. This is a major theme in the book of Hebrews, which was written to the Hebrews by the way.
The Body of Christ does not have any need for a priest at all, in the order of Melchizedek or any other, for we are IN CHRIST Himself! Does Christ need a priest? No! We are His body, we have been hidden in Him. We were crucified in Him and it is no longer we who live! (Gal. 2:20) Does a dead man need a priest? Certainly not!
What, NO! it wasn't to help them understand anything. It HAD to happen on those days because that's what those does had always been about! The Feasts would never have been established in the first place had God not intended them to foreshadow Calvary. That was their primary purpose. Even down to details like the sacrificial lamb must not have any blemishes or broken bones was fulfilled by Jesus on the exact day of the feast. There can be no denying that Calvary was a direct fulfillment of dozens and dozens of both direct and indirect prophecies that were part and parcel of the law!
Note what your cited passage says, "Thus it is written...".
Written where? (In the law!)
The fact that it (Calvary and Pentecost) happened according to Old Testament prophecies is direct evidence that it is not a new dispensation but merely the continuation of one. Nothing was new here except that now instead of looking forward to the sacrifice of God's Son, they looked back. That's really the only difference. Everything else was entirely centered on Israel and their Kingdom. The plan, as I stated before, was for God to send Jesus back and for Israel to get their kingdom and for the whole world to be evangelized through Israel. (There is strong evidence that God had plans for a Body of Christ even with Israel still around but that's a topic for another thread.)
Note that Paul's message was not prophesied...
Romans 16:25 Now to Him who is able to establish you according to my gospel and the preaching of Jesus Christ, according to the revelation of the mystery kept secret since the world began
Galatians 1:12 For I neither received it from man, nor was I taught it, but it came through the revelation of Jesus Christ.
Resting in Him,
Clete