How does Jesus' argument from David and the show bread work?

   Heb 10:1 ¶ For the law having a shadow of good things to come, [and] not the very image of the things, can never with those sacrifices which they offered year by year continually make the comers thereunto perfect.

A shadow of something is not the thing. It is attached to the thing and is revealed by the light. The law has a shadow of the "good things coming". The good thing coming was the cross of Christ. This is a direct statement speaking of the sensus plenior. It is a picture of Christ hidden in the law.

Jesus uses an indirect reference to Uzziah and Azariah as a combined picture of himself. Uzziah represents Christ in the flesh, and the intentional mistake of calling him Azariah, tells us in the language of prophetic riddle, that he would be made the high priest in resurrection. Jesus references a narrative of Ahimelech, but replaces him with his son. Ahimelech represents Christ in the flesh as he was killed for helping David, and his son replaces him as high priest in a picture of resurrection.

The object of the riddle is the Messiah. He is the man in all the riddles. God rested on the Sabbath not because he needed rest, but as a prophecy of his future rest offered to mankind. He gave the law, not because God needed something to enforce, but as a prophetic riddle revealing the nature of God. Any particular law is only a partial revelation of God's character.

But God's character is more important than the law which reveals it. When the picture of his character conflicts with his character, his character wins out.

Jesus is the source of the future rest. The law is a picture of Jesus. Since Jesus is present, the picture has no more effect. This is the thesis statement of the book of Hebrews. What is past is the shadow, Christ is the reality.

   Mt 11:28 Come unto me, all [ye] that labour and are heavy laden, and I will give you rest.

The answer then revolves around how does the bread represent the cross? Jesus said that the bread wash his body given for us. The bread hidden in the temple would be made available to all through the cross. As such, the picture of David eating the bread, and giving it to his men is a prophetic picture of the last supper, and even more so of the cross, wherein we partake of his flesh, and through which we enter into his rest.

Reality trumps the shadow.

The question accuses David of acting dishonestly, but does so through concept feeding in the question. It should be removed from this question and asked directly so that it can be addressed without distracting from the main point here.