I think it is important for one to realize also that the Christian concept of Jesus' sacrifice is an expiatory one, not a propitiatory one. It is not a matter of God needing to be appeased, but a matter of an abstract legal debt being absolved.
Actually it is both. And important enough to send a Son to die. But for what? For the life that was given to the human race in the first place. No one, no animal, the Law, nothing could appease God for the abstract legal dept He suffered but the one that provided this life in the first place, the literal creator of man. Animals could make better human beings of those that had this life with the forgiveness of their sins, and establish this Godly principal but their life (the one common and contiguous feature we all have) was still condemned regardless. This is why the statement you made: "Meanwhile, the Jesus movement universalized the old hopes of a revived, restored Israel into a gospel proclamation that the whole world will, or has been, set right to God through Christ" now became possible. This life was not limited to one nation or tribe. It was common to us all.
Why only Jesus? Paul elaborated on theological grounds the uniqueness of Jesus in relation to God and to man.
Uniqueness? Yes, you can say that. But John said: 3 All things were made by him; and without him was not any thing made that was made. 4 In him was life; and the life was the light of men. That life was the source of that same life passed on to Adam at this beginning. Joh 6:40 For my Father’s will is that everyone who looks to the Son and believes in him shall have eternal life, and I will raise him up at the last day." Joh 6:47 I tell you the truth, he who believes has everlasting life. So many examples we simply overlook. And we now learn that this life can be selectively redeemed and restored. A word like "life" we think we know what it is and may gloss over it without realizing it’s significance to God. Yet who understands life or can create life from the dust of the ground?