I'm not trying to take sides in a debate here so as to thereby fuel further contention; just trying to help clear up what seems to be an honest misunderstanding between good persons:
It does not seem to me that GMK in any way contradicted himself. Riddikulus’ comment regarding the NT 'trumping' the OT was made in a context which clearly suggested that she believes it did so in the sense of 'correction'. GMK has clearly, and, in my opinion, rightfully, opposed the idea that the OT was corrected by the NT. He ascents to the vague assertion that the NT 'trumped' the OT insofar, and only insofar, as that means that the OT was fulfilled/completed/perfected by the NT—which is no doubt the sense that the Lord Christ intended his comments relevant to the relationship between the two Covenants, as a patristic and academic analysis of those comments would elucidate.
I think that by suggesting to Riddikulus that use of the OT by the NT Church makes no sense if one were to assume that the OT was false in its presentation of God, GMK was trying to make a valid point. It would certainly make sense for me to publish the Prologue of a certain work upon the completion of that work, but it would be silly for me to publish the rough draft, replete as it is with errors, along with that completed work.