The RCC teaches that the RC Mass is a carrying on of the work of salvation effected by Christ. Why are the 'merits' that Jesus received by His sufferings and death on the Cross only applied through the Roman Catholic Sacrifice of the Mass and how are they applied and who applies them?
Because the Sacrifice of the Mass is the Sacrifice of the Cross; just under a different mode of presentation.
The Sacrifice on the Cross was bloody and fleshy. The Sacrifice on the Cross was also offered by Jesus in his unglorified flesh. Hence, the sacrifice lead to suffering and death. Note, but the way that Jesus IS the sacrifice. The Cross was just the means by which he offered himself unto death.
In the resurrection, the sacrifice is glorified. Jesus flesh is no longer earthy; thus Jesus is now able to offer himself eternally to the Father as the complete sacrifice without suffering and dying. Jesus can pour himself out as the Lamb that was Slain without dying. This is what makes the Mass possible. Now the sacrifice of Christ is presented in Sacramental form; it is the same sacrifice, just not under the mode of earthly flesh. This is why Catholics are not cannibals when they receive communion. The flesh that is received is not earthy flesh. The flesh that is received is not consumed and broken down in the body. The flesh received is glorified flesh and thus is not subject to destruction. Those who accuse Catholics of cannibalism, those who think Catholics believe that Christ is "re-sacrificed" at the Mass are thinking in terms of the flesh; as Jesus tells us, the flesh profits nothing. If you want to understand this mystery, it is necessary to put on the mind of Christ and stop thinking according to the ways of the flesh.
When Christ uttered the words "It is finished" he wasn't referring to his sacrifice. He was referring to his earthy work and his earthy life.
The merits of Christ are infused in the believer through Baptism. This is what gives them a participation on the sacrifice of Christ. Baptism is what gives the Christian the right to celebrate the Banquet feast of the Lamb, the Mass and participate in it and receive the Body and Blood of the Lord.