This was a red herring. Somehow I loaded the libusbk driver for the Spark Core AND the CORE DFU (flashing yellow mode) when it should have only been loaded for CORE DFU. Symptom of this is in Windows Device Manager the Spark Core appears under “libusbK USB Devices” instead of under “Ports” where it should be. This is what bad looks like:
Solution is to reload driver using “Update Driver Software…” portion of instructions from here.
This is what good looks like:
Now back to my regularly scheduled problem.