Draining the irrigation valves shown - confused about which side is inlet and identifying components?

Flow direction will be engraved on the side or bottom. Stick your phone in selfie mode down there to see it. Alternately clean off that red plate between D and E, you'll see the model number and can look up the manual on line.

Draining will only help if this is the low point of the whole irrigation system and the pipes generally run uphill with no low points. Turn off the water supply, open ALL those valves and all the zone valves, and hope for the best. There might be another drain valve near the supply cutoff. Open that too. This isn't reliable.

The preferred method is to use an air compressor to blow out the water. If you or a friend have a large compressor. If you can carry it it's too small. At a high level you connect the compressor to the downstream port above (maybe A, but you need to confirm), one at a time open the sprinkler zones, and blow them for a couple minutes each. These aren't complete instructions, if you are going to do that you should watch some youtube tutorials and read manuals.

You might find a landscaping company that will come do this for not too much money.

I'm surprised to hear that a freeze in Austin would be cold enough and last long enough to freeze the ground right through to the pipes and the water in them. Didn't know that. I'm in NJ, I have a landscaper come do this each November with a huge construction compressor. He charges much less than I would have to pay to rent the thing for 4 hours.