There are many Bible verses saying that Jesus is the mediator between man and God. 1 Timothy 2:5,6 , Hebrews 9:15, 28 , Ephesians 1:7 , etc. There is no single verse that could even point that there should be a mediator between common man and Jesus.
Even if we accept the parable of "faithful slave" says what the GB tells us it says, that offers no basis of mediatorship. The slave was there only to "dispense food", not to reconcile the other slaves with their master. On the other hand, that same passage talks about an evil slave, who put himself above the other slaves.