Greetings!
I think the problem is in the use of the language. At times very limited meanings of the term "mediator" are used and at other times looser meanings are applied.
Normally, we tend to use a very broad meaning that "mediator" means simply a go-between. Someone that serves as a third-party facilitator or who is the means by which something is accomplished between two parties.
Thus when we talk about PRAYER, we view Jesus as between us and Jehovah God and thus sometimes refer to him as a mediator of that situation. Jesus is a mediator (or more accurately a facilitator) for ALL mankind when it comes to prayer to Almighty God.
Similarly, when we look at Grace (or Undeserved Kindness as JWs call it) and Salvation we understand that it is because of Christ's sacrifice, the value of his life, that he is able to present to Jehovah God in our behalf, that by means of that sacrifice our sins may be forgiven and we may receive Salvation through the Grace of God. Thus in this since Jesus again is a mediator for all mankind in this regard.
The Society has a very limited meaning of "mediator" when it comes to the NEW COVENANT, that is the promise to those 144,000 that they will serve as Kings and Priests in Heaven (as a ruling class) (this is of course traditional JW theology not my own private belief).
Thus when it comes to that CONTRACT or COVENANT, they mean that Jesus is the attorney (aka mediator) that will argue before Jehovah God the merits and terms of the contract on behalf of the beneficiaries (the 144,000). Because, according to traditional JW theology, this contract or covenant is between God (Jesus as God's agent or representative on Earth is the one that actually made the contract) and only the 144,000, when it comes to talking about this situation the Society says that Jesus is thus only the mediator for the 144,000 which makes sense.
So you see with regard to the above there isn't really any contradictions or conflict with the scriptures. It is just a problem of what we mean when we are talking about Jesus being a mediator and what the situation is. So both you and your mother were correct in a sense.
-Eduardo Leaton Jr.
PS: When it comes to FORGIVENESS FROM SINS, CONFESSION, and a sinner reaching SPIRITUAL HEALTH again, since June 1, 2001, the Society has set up ELDERS as an additional layer between God and Jehovah's Witnesses and thus in my view they have violated the Scriptures and the fact that only Jesus should serve as a mediator between God and Mankind.
See this info:
[excerpt of entire article] Questions From Readers
In view of Jehovah's willingness to forgive sins by the merit of the ransom sacrifice, why is it necessary for Christians to confess to the older men in the congregation?
As can be seen in the case of David and Bath-sheba, Jehovah forgave David's sin, grave though it was, because of David's genuine repentance. When the prophet Nathan approached him, David openly confessed: "I have sinned against Jehovah." - 2 Samuel 12:13
However, Jehovah not only accepts a sinner's sincere confession and extends forgiveness but he also makes loving provisions to help the erring one progress to spiritual recovery. In David's case, the help came through the prophet Nathan. Today, in the Christian congregation, there are spiritually mature older men, or elders. The disciple James explains: "Is there anyone [spiritually] sick among you? Let him call the older men of the congregation to him, and let them pray over him, greasing him with oil in the name of Jehovah. And the prayer of faith will make the indisposed one well, and Jehovah will raise him up. Also, if he has committed sins, it will be forgiven him." - James 5:14, 15.
Skillful elders can do much to ease the pain of heart that is felt by the remorseful sinner. They strive to imitate Jehovah in their dealings with him. They never want to be harsh, even though strong discipline maybe warranted. Rather, they compassionately consider the immediate needs of the individual. Patiently they strive to readjust erring one's thinking by using God's Word. (Galatians 6:1)
Even if a person does not voluntarily confess his sin, he may still be moved to repentance when approached by the elders, as David was when approached by Nathan. The support thus rendered by the elders helps the one to avoid the danger of repeating the sin and the serious consequences of becoming a hardened practicer of sin. - Hebrews 10:26-31.
It is certainly not easy to confess to others deeds that one feels ashamed of and to seek forgiveness. It takes inner strength. Reflect for a moment, though, on the alternative. One man who failed to reveal his serious sin to the elders in the congregation said: "I felt a pain in my heart that would not go away. I increased my efforts in the preaching work, but the sickening feeling remained." He felt that confession to God in prayer was enough, but clearly it was not, for he experienced feelings similar to King David's. (Psalm 51:8, 11) How much better to accept the loving assistance that Jehovah provides through the elders!
The Watchtower, June 1, 2001, pg. 31]
, as of June 2001, and largely without any notice or concern on the part of the majority of Jehovah's Witnesses, the Society has instituted what it claims does not exist: the separation of a "clergy" and "laity" class and the institution of persons serving as an intercessory before God. It has now vested congregational Elders with the role of "mediator" between a person and Christ Jesus in direct contradiction to the Scriptures. If any Witness doubts that this is not the case all such one has to do is to approach a local elder and pose the question of whether a baptized publisher must make confession of a serious sin to the Elders or whether it is possible for such one to obtain complete forgiveness and spiritual healing from God alone without having to confess to the Elders.