To close out this discussion, the system is once more operational. Turns out that the panel I received from KY was probably good but the techs were not aware that the telephone-type control wire required a mod (rolling connector on one end). After installing a bran-spankin-new remote panel, everything is fully functional now.
So, if you encounter such an issue, rest assured that the manual is lying when it shows compatibility between the new inverters and the old RC6 (and probably others) panels.