Maybe I've missed it, but I haven't seen a chlorine level reading in the late PM, followed by another reading the next early AM.

Those two readings TOGETHER are what gives indication of what's happening. A chlorine reading -- with no time stamp -- followed by a chlorine dose -- with no time stamp -- followed by another reading the next day -- still with no time stamp -- is not particularly informative.

Also, you write that a gallon per day is not sustainable. At Walmart, fresh bleach is under $3/gallon in most locations. For a 120 day season, that's $360, which is much LESS than most pool owners spend on their pools during a 120 day season, when following pool store recommendations. So while I'm not advocating a gallon per day as normal, a whole lot of pool owners sustain even higher expenses.

Let me be more specific.

1. Raise your chlorine level to a tested 15 ppm or higher at 7 pm.
2. Add nothing else. Make sure your pump is on all night.
3. Test again after sunrise, but before 8 am.
4. Report BOTH results.