After some sleuthing, I figured out the culprit for why the Riffle was consuming so much power, as detailed in this Research Note.
Turns out there is a bug in the SD Card Library included with the Arduino IDE that doesn't let the SD Card sleep.
I found this thread in the Arduino Forums, which says this about the SD Library included with the Arduino IDE.
SD.h has not added bug fixes from SdFat for about three years and the Adruino developers have added their own bugs. I tried SD.h with my tests and found cards pulled from 15 to 28 ma when idle.
That was posted 3 years ago, and is about what I was experiencing with the Riffle.
As suggested in the thread, I switched over to using the sdFat Library and the sleeping current consumption dropped to about to 0.5mA, which is what the literature suggests. The Riffle sleeping consumes about 300µA, and the SD card consumes about 200µA.
I also discovered the Open Source Building Science Sensors project. They are using the sdFat Library to successfully and stably power cycle the SD card itself.
I was successfully able to power cycle the SD Card using the MOSFET switch included on the board. Not sure if it really makes much of a difference power wise....but it seems pretty stable over the last hour at a logging interval of 15 seconds.
Updated code is in the Github Repo.
Currently, the sleeping current is 0.487mA, which is a theoretical battery life of about 3-4 months depending on logging interval.
Better than 9 days!