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I am about to embark on a Chapter dealing with the soteriology of the Church's first few centuries. When you look at my overall Draft Study, you will see that I have drafted a Chapter on Anselm, who dreamt up the idea (based on his medieval culture) that Jesus' death compensated for the death that a believer should have experienced because of their sin.
Prior to Anselm, the Church followed Origen, who said that the death of Jesus was a ransom payment to Satan. With the Watchtower's "ransom" theory, I do not know who they believe the "ransom" was paid to.
Regarding the idea of the worth of Jesus' "blood" that you raise, again it is a matter of understanding idiomatic usage, as you point out. In this case, the term "blood" simply means his "death". With Jewish writings, poetry consists of paralleling ideas, whether as agreements or as contrasts. Off-hand, I cannot recall the precise reference, although I think it is in Romans, where the term "blood" is paralleled with "death".
Note, however, that this is Paul's take on salvation. His focus is totally on Jesus' death and resurrection. His need to emphasise this shows there were others who had a different focus. I think here of the Johannine community, which wrote that God gave his son (speaking of the birth, not the death) so that any who believes will be saved. John focuses on the life of Jesus ("I am the way", etc., which Jesus probably did not say) not on his death. John does not places the "this is my blood/body" within the context of the Passover.
You will see my understanding of the Johannine community at my Draft Study.