Beware a solid pool cover leak!!!

I have two automatic pool cover pumps that sit atop my 20,000 gal in ground. One of them has a base with rounded corners and turns on automatically every five minutes and will pump out any water it senses, but the other one, a Little Giant, a sump pump activated by water pressure, sits on a thin plastic platform with rather sharp edges (it was advertised as used in pool cover water evacuation as I recall when I bought it a couple years ago). Sometime Sunday afternoon, I figure, a pinhole leak developed below the Little Giant (although I cannot see it standing at the edge of the pool). I got up at 3:00 a.m. Monday morning to let the cat have some backyard time because we had a 100% chance of rain Monday and she would have to stay inside (she hates it inside but she's 17 so I'm very careful with her). I turned on the flood lights and then saw it: my pool was concaved in a major way. It's the stuff nightmares are made of. I could only wonder about the status of the older liner and if it would hold up (I think it did fine). Because the brand spanking new cover is black, I couldn't tell how much water was missing, but guessed it was somewhere around 30-40%. I removed the Little Giant (I knew the problem was under that one because I saw the discharge garden hose spitting out water in the lower yard - and it hadn't yet started raining, we've been in a historical drought here and hadn't had rain in many weeks), stuck the garden hose under the cover, turned it on full blast and went back to bed -- nothing to do until first light. It took until 9:00 a.m., or six hours, for the water level to get back up. I turned the pump/filter on and crawled under the cover and dumped enough bleach in the skimmer to raise the FC to 12 ppm. For the heck of it I checked my CYA and it was down to about 28. I won't worry about adding more until the spring because I don't believe it's a factor in keeping chlorine when you have no sunlight?

Which brought me to a question I have pondered before. We're all familiar with Ben's 'best guess' FC/Stabilizer table which advises minimum and maximum chlorine levels according to one's CYA levels. For example, with a CYA of 30 the maximum CL is 6 ppm, but you shock to 12. Because we customarily shock our pools right before we cover or close in the Fall, and because there's nothing to eat up the chlorine with a solid cover (i.e., sunlight), that high shock level stays for months on end. I know because I usually test my water mid-winter; even in the Spring, 8 months later when I uncover, it's usually around 8 ppm. So if the maximum chlorine level is 6 ppm, presumably so one's liner doesn't bleach out, what about that shock level of 12 staying high for so long? I have pondered on this forum years ago why my liner bleached out so much, and I can't help but wonder if this is why? If a prolonged ppm of 12 will eventually bleach out one's liner, then this may answer that enigma that has totally eluded me.

Another thought: anytime you ostensibly could spring a cover leak the same thing could happen, so maybe it's a better idea to keep the automatic pump(s) off, i.e., not plugged in, unless home to monitor? The downside to that would be if you were away at work for 10 hours and got a massive amount of rain it could be enough to displace via the weight of the rain your pool water? I wonder how much it would displace? Maybe if it's less than having a cover leak and an auto pump pumping it out, it would be more prudent to keep the pump(s) unplugged?