It has been a while since I posted on this. The project shows a lot of promise.
1. Radon levels of 4.0 pCi/L is where remediation is recommended. Within a day of installing the ~$140 radon fan, my level dropped from 12 pCi/L to somewhere between 0.1-0.4 pCi/L, depending on the day. I thought I would need to fix many more things to get the radon to acceptable levels - like plugging 3-4 gaping holes in the slab that are currently serving as drains, but the fan got me there immediately.
2. I was able to take some temp and humidity measurements before winter ended, when some nights were still below freezing and some days got up into the 60s. The cold air inlet / warm air outlet venting were only half complete, but temperatures were stable and hung in the low 40s +/- 2-3 degrees, and humidity remained above 70%. This is for the entire 22'x22' area - I expect that I'll have more control when air flow is confined to a very well insulated 22'x4' area.
Below are 2 pics - a close up of the venting and a wider shot with shelving. I am using the cheapest 4" drain pipe and harbor freight dust collection regulators to manage air flow. I'll have to wait until next fall to know for sure, but I think it's going to work great. Next comes the walls and insulation. Then lighting and some kind of insulated door. After that I'll get a couple hundred pounds of root veggies and see how they last into 2020. I'll be able to monitor temp/humidity outdoors and inside the cellar and will post a few graphs to show how well the thermal mass of the cellar dampens the temp and humidity swings outside.