"Are there no remote reading instruments which will work with the Avidyne kit?"
Not that I know, the dual box is the only option as far as I am aware, the ADF needle would be "imposed" on the MFD I believe.
A and C, I could put the KN63 in the back and the KDI572 on the left of the PFD, but that still leaves me with the ADF RHS cutout, so hence the cheaper KN64 option.
It seems daft that there is no other solution after all these years.