I got this one though.
I had misread the line "What I have to do to FORCE the transmit is to tune the Rx band down 5 to 10 kHz so that the TNC no longer sees packets - then there will be a flurry of Tx activity, then I quickly tune Rx band back to the proper frequency to resume decoding the downlink." From that, that's where I went with rig control, etc. Sorry about that. Anyway, in RE-READING that line, it sure seems like the TNC really isn't in full duplex and it's waiting for the channel to be not-busy before transmitting. Aka.. acting like it's a vanilla TNC on one frequency using one VFO.
I'm curious if your D710 is fully updated with the newest main and TNC firmware? It's worth checking. Beyond that, this smacks of a possible TNC bug. I understand that FULL DUPLEX actually inhibits the DCD detection automatically. Just guessing but maybe if you put the DCD setting back to default, maybe the full duplex behavior will start working as expected?