I think it's time to let it drop.
I haven't tested enough to be certain, but I suspect that, when FC levels are very high, spurious CC levels may appear. If your FC is stable overnight -- and 2 ppm is fairly stable.
There may be some lingering chlorine demand. However, the best way to deal with it is probably simply to run FC levels equal to 10% - 15% of your CYA for awhile.
Black ring around the skimmer can be from body oils and lotions OR it can be from plasticizers being released from the liner. If it's plasticizers, there is NOTHING you can do about that, except keep the pH reasonable (between 7 and 8).
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