Just a point about Russell and Jesus as Mediator:
Like the WT today, Russell taught that Jesus could only be the "mediator" in a covenant relationship between God and his people. One would think that Russell would teach that all Bible Students were under the New Covenant because they all had the heavenly calling back then. But this was not the case. Particularly from 1909 onward, Russell taught that the New Covenant would be initiated with natural Israel after Armageddon - that the Church was never under the New Covenant. (Some scholars claim Russell always believed this, but merely re-emphasized it in 1909. The original prophecy about the new covenant, in Jeremiah 31, does appear to have a Jewish context.)
So, Russell reasoned, if the Church is not in the New Covenant, Jesus could not be the mediator for the Church. (Instead, he would be the "intersessor.") Well, this caused quite a stir, as there were Bible Students then who were just as offended at the thought of Jesus not being their mediator as some of the "other sheep" are offended today. This resulted in what is known as the "New Covenant Schism" - the breakaway from the WT of Bible Students during Russell's own lifetime to form a separate group. These people's descendants are still around today as the "Free Bible Students," and they stopped using the SCRIPTURE STUDIES way back then!
Of course, today the WT teaches that the spiritual Israel (the Church, the anointed class) is under the New Covenant, that natural Israel will not be restored after Armageddon, and that the majority of the current JW's (who are "other sheep") are still not under the New Covenant. What has remained consistent is the reasoning that the mediatorial role of Jesus only exists within a covenant, and that those not in the covenant have a different legal relationship to Jesus, and hence to Jehovah.
Justin