Update... I had my installer re-do most of the wiring.. there were some errors with the low-voltage setup. Ultimately, the cause of this entire mess was that one of the sequencers was bad (probably from the start) and would not turn on its element, or the one downstream element, so that only one was running on auxillary heat.
The sequencer was replaced and the problem corrected.
Thanks everyone for your help. I am slowly defrosting.