OK here are the results of the same test again, except this time I'm using cellular data only. I kept WiFi off the entire time. I took the phone off the charger at 100% about 1 day, 1 hour ago and battery is now at 24%.
I tried for similar usage of the Signal app as the first test, stayed at home like before except for taking the same walk in the same park as before, taking some more pics and sending them off to people.
Sent 14 messages (8 w/attached photos)
Received 11 messages
Had 1 voice call lasting 13 minutes
Battery usage for Signal:
Used for 1 day, 1 hr and is at (a whopping) 20%
So yeah that's terrible.
Top battery users:
Phone idle: 13%
Android System: 1%
It turns out that relying on the cellular connection alone made me pay attention to things, and I learned that the cell signal strength inside my house is crap. Wandering around inside my house the best reading I saw (under Settings > About phone > SIM status (sim slot 1) > Signal strength) was -106 dBm, and that was near a big window. Most other places bounced around between -117 and -110 dBm. I think that's pretty much bottom rung.
I don't know what the signal strength for my WiFi connection is in dBm, but in Settings > Internet > [network name] > Signal strength it says Excellent. Back into airplane mode + WiFi mode I go.