This time, I tested the longevity by charging to full, turned off all three killswitches, set the phone aside, then only activated the screen occasionally to check the charge (except when I was sleeping).
The battery drained from 100% to 3% in just over 14 hours.
My final check of the power level at about the 14-hour mark actually showed a remaining charge of about 35%, but that was apparently inaccurate, because a few minutes later I noticed that the phone had shut down. When I turned it back on, the level was at 3%. So the charge indicator is still way off, as has been explained before.
I have this phone on WiFi most of the time, with cellular sometimes on and sometimes off, and I’m getting from 6 to 8 hours comfortably, depending on what I’m doing with it.