The diet cola experiment was a success. The raw data over 2.5 days shows the daily peak-peak variation dropped almost 10X:

But there are still periods of jiggyness(?):

The artifacts that have waveforms that start with large amplitudes and tail off are probably small magnitude quakes. The artifacts that appear to be ball-shaped are apparently caused by wind. The sharp spike around 2019-07-16 00:00 is probably a nearby lightning strike. I was monitoring the weather outside over this period (except at night) and found that the jiggyness correlated closely with an uptick in the windspeed. The times indicated above are UTC (00:00 UTC is 6:00pm MDT). That last bit of jiggyness occurred around 2:00am MDT and I happened to be awake enough to hear it. 

I did not see these artifacts with the AD7745 because the noise floor is higher and the filter BW is now 0.01Hz - 0.5Hz vs. 0.025Hz - 0.5Hz for the AD7745. Also, the wind tends to increase around the same time of day lately, which corresponds to the transition from rising to falling in the raw waveform. This led me to assume that it was somehow related to temperature. Another interesting item: the wind seems to cause the house foundation to tilt, which is evidenced by the flattening of the raw data curves at the peaks. 

Moving On:

I took the seismometer electronics apart and installed the latest C2D board with a 6-inch ribbon cable, shielding and foam insulation. I located the sensor inside the seismometer enclosure and ran the ribbon cable outside the enclosure to the RPi-B+. This should keep the RPi-B+ from causing temperature variation inside the enclosure. The ribbon cable running under one side of the enclosure may cause a small air leak, but I'm hoping that the cola cans absorb it. So far, everything appears to be working but I'm not getting the 21.5 bits of dynamic range that I saw on the bench:

At this point I don't have any explanation as to the decreased performance when the C2D is coupled to the sensor. It is 3-4X better than the AD7745. That will have to be good enough until I have the time and ambition to investigate further.