Caracat, with these recent posters, you’re in good hands. They’ll help you get it figured out.
I was thinking about you this morning at the time I needed to get my pool water sample (about half an hour after the pump has started). It was pouring rain, so I didn’t want to get it then. Do you have a rain gauge to know how much rain you get at a particular time. Normally rain doesn’t have too much effect on chemistry, but the rain our area of Texas has been getting has been anything but normal. I haven’t been checking on Houston, though. Rain water in significant quantities might take some time to mix with your pool water, so it could throw your testing off a bit if it has rained torrentially and not mixed thoroughly. Probably does not account for everything going on in your situation, but I was thinking about it as the rain came pouring down on my pool.
Also, I was looking back at your oh-so-detailed test results, and something caught my eye. In what I was reading you give an evening test time and then indicate the amount of chlorine you add to get to 24. The next timed result is the morning. You are testing again after the pump has mixed in the last addition of the evening, right? That’s an important step, and I didn’t see it recorded in what I looked at, but I didn’t reread your entire log.
Also, just a heads up that old main drains can be dangerous. I wouldn’t send your eleven year old down there without knowing whether it’s running or not and how much suction there is. I’m afraid I have no expertise to offer. I just know you should be cautious.