Then Mark 2 does not really mean: "Who can forgive sins, but God only?" (emphasis added) Instead, it really means: "Who can forgive sins, but God only, or God's human designee." I see a significant difference between these two.
Who are you to take away God's Freedom to designate men to provide visible manifestations of God's graces among us? Can you show one verse that PREVENTS God from dealing with men in such a way? I suggest you read the Matthew version of this story - the JEWS THEMSELVES recognized with awe that God had given men the power to forgive sins! Did God or did He NOT give men the power to forgive sins in John 20? Only twisting the Scripture out of recognition will bring you to falsely think that God is "not allowed" to deal with men in such a manner.
And secondly, of course, God STILL DOES forgive sins...The priest is "in the person of Christ". The priest visibly represents the voice of Christ. Catholics KNOW they have been forgiven of sin. Protestants always wonder "did my sinner's prayer take? If I start to fall away, I might never had been saved (healed) to begin with!"
Regards
You need to look at it in light of Matthew 16:19 (and 18:18): "And whatsoever thou shalt bind upon earth, it shall be bound also in heaven: and whatsoever thou shalt loose upon earth, it shall be loosed also in heaven."
In this you have Christ's promise to lend His Divine support to the acts of the Church.