Hi serein
The mediatorship is the specific covenant dealing with the firstfruits that will be acting as kings and priests in heaven.
Romans 8:17
Now if we are children, then we are heirs—heirs of God and co-heirs with Christ, if indeed we share in his sufferings in order that we may also share in his glory.
Revelation 14:4
These are those who did not defile themselves with women, for they kept themselves pure. They follow the Lamb wherever he goes. They were purchased from among men and offered as firstfruits to God and the Lamb.
Romans 11:16
If the part of the dough offered as firstfruits is holy, then the whole batch is holy; if the root is holy, so are the branches.
It in no way stops us praying to Jehovah in Jesus's name whatever our hope whether it is earthly or heavenly.
We as witnesses do not preach that only the 144,000 can pray to Jehovah through Jesus so You are mistaken on our bible drawn beliefs there.
Jesus is priest to all mankind having sacrificed his blood for us all so we all go through him.
Hebrews 9:24-26 (New International Version)
24 For Christ did not enter a man-made sanctuary that was only a copy of the true one; he entered heaven itself, now to appear for us in God's presence. 25 Nor did he enter heaven to offer himself again and again, the way the high priest enters the Most Holy Place every year with blood that is not his own. 26 Then Christ would have had to suffer many times since the creation of the world. But now he has appeared once for all at the end of the ages to do away with sin by the sacrifice of himself.