If you want to understand what Mormons normally teach regarding the role of Jesus Christ as mediator, watch this video made by Latter-day Saints. You will see how they themselves understand Jesus' mediation. Here's a sample of the main conversation in the story:
Mediator: I will pay the debt, if you will free the debtor of his contract, so that he can keep his possessions and not go to prison. You demanded justice. Though he cannot pay you, I will do so. You will have been justly dealt with, and can ask no more. It would not be just.
(Mediator Turns to Guilty Prisoner)
Mediator: If I pay your debt, will you accept me as your creditor?
Mediator: Then you will pay the debt to me, and I will set the terms. It will not be easy, but it will be possible. I will provide a way; you need not go to prison.
Additionally, don't miss the summary of the commentator:
And by eternal law, mercy cannot be extended, save there is one [Jesus Christ], who is both willing and able to assume our debt, pay the price, and arrange the terms for our redemption.
So, here is the fundamental question: does Jesus set and arrange the terms for our redemption, or does He actually accomplish out redemption?
Hint: Notice that they use the word mercy, but not grace.