My mother is a CO's wife. When I brought this up with her she called me a liar and refuses to believe this is a Watchtower teaching.
A classic article is Watchtower 1989 August 15 pp.30-31 Questions From Readers. This basically says that you can not understand the mediator doctrine if you do not understand other Witness doctrine, a true indicator of the Watchtowers Eisegisis approach to interpretation.
Here are a couple of other quotes
Insight on the Scriptures, Volume 2 p.362 Mediator
“He mediates the new covenant between God and those taken into the new covenant, the congregation of spiritual Israel. …The total number of those who are finally and permanently sealed is revealed in Revelation 7:4-8 as 144,000.”
The Watchtower states that whilst Jesus is mediator for 144,000 only, those not under the new covenant will be blessed by Jesus through his role as their High Priest. That is why the great crowd are allowed to pray through Jesus, rather than through the 144,000.
Insight on the Scriptures, Volume 2 p.362 Mediator
“ Blessings to Mankind in General. While Jesus’ mediatorship operates solely toward those in the new covenant, he is also God’s High Priest and the Seed of Abraham. In fulfilling his duties in these latter two positions, he will bring blessings to others of mankind, for all the nations are to be blessed by means of Abraham’s seed.”
Watchtower 1991 February 15 p.18
“Christ does not act as Mediator of the new covenant toward them, yet they benefit from this covenant through the work of God’s Kingdom. Christ still acts toward them, however, as High Priest, through whom Jehovah can and does apply the ransom to the extent of their now being declared righteous as God’s friends. (Compare James 2:23.)”