Good news to report I think! Apologies for the long gap, but I've been still working at the pool as able and I had some family staying at the house and they were helping take care of the water too. A few days after my last post, I went by and suddenly the water was by far the most clear I'd seen. Clear enough to see the bottom liner pattern in the shallows and dark patches of collected junk in deeper spots. There were a couple of large accumulations (the worst at the bottom of the slope to the deep end) and some dusty-looking material stuck on side walls around the deep end. I did thorough vacuuming over several days that got a vast majority of the junk. Some of the stuff down there now I am sure is newer from nearby trees. I have been having some difficulty getting small sticks/twigs and larger leaves out, it is hard to get them with the vac since they float out of the way and will only get sucked up if you cover them dead on with the vac port to get enough suction (and spending a minute to get one leaf or something wastes a lot of water). I ran the previous owner's pool vac and it worked great, but now won't start up after that first cycle (just my luck). For now I picked up a cheap battery robo-vac and it seems to be doing decently, at least on the large debris (doesn't have an actual filter element like the other vac).
I am basically sure there is a leak now though, I put tape and measured it and it dropped 1-1/2" in 24 hours while the bucket only lost about 3/8". I don't think it was leaking this bad earlier in the summer, because I would have picked up on it.
As far as the water, it's gotten pretty clear now and you can see detail even down in the deep end. There might still be a little room for improvement, but I'm pretty pleased with how it looks so far.
I think if this holds and passes the OCL test, it should be time to lower the FC level. I'll then try to balance out the rest of the water chemistry. I'm trying to decide about what to do at this point and whether to try to add salt and get the salt water cell to come on and stabilize, since we'll probably have to start closing the pool up in a few weeks.
With all this trouble so far I have not looked into the closing process much. We have a pretty warm climate (north Alabama), so I think there are things you can get away with here and not have to go through as much detailed freeze protection and stuff like that.