Awesome info! So glad I won't have to take off the fixed cog after all, it will be a lot easier on me to use the threads on the other side of the hub. So apparently the hub on this bike is still strictly speaking a "flip-flop hub", because it's threaded on both sides, and they used to ship this bike with the freewheel attached, but no longer.

There are already brakes on the front and back, so I am good there.

I went ahead and measured the diameter of the threads, they are very nearly 35 mm in diameter. I assume this is the standard/default size? I don't see any mention of diameters when shopping for parts.

I am not confident in my ability to distinguish between the 1/8" vs 3/32" chain, so I sent a note to nashbar asking them to clarify.

Just looked closer at the specs for the bike listed on the nashbar website and it identifies the chain as "CHAIN KMC Z410", which according to google is 1/8". So I went ahead and ordered this Shimano SF-1200 freewheel with 16 teeth. Hopefully this will do the job!