I know Ray Franz is hero to many here. I knew him and his uncle. Fred was nuts, Ray I never liked. Both could present themselves in a pleasing way. Fred knew my mother and other relatives. My first memory of him is from when I was a little boy. His version of interacting with children was to poke at them and make stupid noises. Neither man was without flaws. Ray picked what he saw as a "hot button" issue and ran with it. He knew as well as I what the Watchtower teaches on the New Covenant and why.
The fact remains, within Watchtower teaching, and within the plain meaning of Mesites, the Greek word, a mediator mediates. The New Testament presents Christ as Mediator of the New Covenant. He is covenant mediator in the same sense Moses was, though of a New Covenant and not of a restatement of the old covenant. If there is a sin covering element, then it is between the parties to the covenant. Witnesses see the New Covenant as limited to 144,000. I doubt this. But within their narrow view, Jesus is mediator between that small number of humans and God. It is an organizational covenant such as the law covenant was. The salvation of all men is dependent not on the New Covenant, but on Christ's ransom. Christ's ransom, according to the Witness view, is a sacrifice for sins. He is both sacrafice and high priest. For Witnesses this is a different function than mediator. They fall within what 19th Century German Evangelical writers taught on this issue. I suspect who ever first formulated this read Lang's Commentary in English translation, and elaborated Witness doctrine from a few scattered comments found therein.
Christ as "propitiator" and High Priest applies it to all. How long were you a Witness so that you do not know this?