how to drain a single loop in one of my heating zones?
You can blow the water out with a purge gas, such as nitrogen, or consider adding glycol to your water system as an anti-freeze.

Below is a picture of Propylene Glycol:


Pay attention to manufacturers recommendation. Also, by the looks of your pictures, I would have an alkalinity test done on your water. That build up around the joints looks like acid. If it is acidic, then your copper will start to deteriorate and cause leaks.