How to isolate a baseboard heater circuit, drain, purge?
You are on the right track. Assuming you want to work on a single zone, you will need to:


Shut off your boiler using the switch (potentially you could disconnect just one zone and leave your boiler running for the sake of other zones, but in general it's simpler to just turn off the system unless you're doing extensive work during the winter).
Disconnect that zone by closing valves on both the supply and return ends of the zone's loop. It looks like you have valves on each of the zone supply lines, but only one valve on your return, after the zones are joined - the pink circle in your last pic. So you may end up draining a bit of water from other zones. Note that if you didn't have valves on both ends, you could also drain your entire system. To do that you'd want to also shut off the cold water supply into your boiler, which is always there to allow the system to fill and respond to evaporation/leaks.
Prepare to drain by attaching a hose/bucket to your chosen zone's drain valve/spigot. Open the drainage valve. You'll get some water out at this point, but it will be limited because there is no other opening to allow air in.
To facilitate draining in the areas you'll be working, open bleed valves on your radiators upstairs. The bleed valve is a smaller valve designed to let you release air from your pipes so that the there is no air in the pipes along with the water. In this case you're going to use it to allow air in instead, so that the water can drain down into your bucket instead of spilling all over your floor when you cut a pipe upstairs.
Make your changes upstairs. Note that if you change the layout significantly you may need to add bleed valves to facilitate purging air in the future.
Close all your bleed valves. Close drainage valves and remove hose/bucket.
Reopen all the valves you used to disconnect your chosen zone. That zone will now be filled/under pressure, but with a significant amount of air in the pipes so it won't yet be ready to run.
Bleed the air from the system. Note that you may need to bleed radiators in other zones as well, because it does not appear you can fully isolate the chosen zone on both the supply and return ends. When bleeding the radiators, open a valve to release air and close it when it stops sputtering. Use a cup and rag to avoid spraying water everywhere. :)
Check again for leaks. :)
Turn the system back on and test it out!


Good luck with your project!