I don't understand why John 20:23 is not sufficient for you. Can it get any plainer that that?
Speaking just for myself, I will admit that this one is more difficult to interpret as to "match" the rest of my theology. I don't feel secure enough to pronounce a slam dunk, as I would in other cases. :) But ultimately, we have a plain meaning apparent contradiction between this and Mark 2:7. Something has to give via interpretation. Naturally, we will both choose the interpretation that matches. And, I think that to an unbiased judge, a credible case could be made for either view.