I think the 15 ppm FC will work fine even with 50-70 ppm CYA. You can measure the FC upon opening next year to see how well it held up over the winter. If it's high, then you won't need to close as high.

You have a Nature2 cartridge for a pool? Ditch it (leave the cartridge holder empty). It's obviously not providing any benefit preventing bacterial growth and the metal ions it releases can stain pool surfaces if they get high enough to do any good and if the pH gets too high for whatever reason (which can happen over the winter).

If you wanted to have any form of supplemental algaecide, then use 50 ppm Borates in the pool. I don't know if that's enough to prevent bacterial growth -- probably not -- but it will slow down algae growth and at colder temps might be enough to slow down bacteria if the FC went to zero for whatever reason.

In my own pool I had some metal staining over this last winter due to iron that is still getting into the water from some stainless? steel mounts that got rusted when Trichlor pucks in a floating feeder parked nearby 7 years ago. The acidity of the Trichlor rusted the mounts (they are for holding stainless steel bars that are just under the waterline). I cleared this up with an ascorbic acid treatment and metal sequestrant (lowering the pH back down from when it rose over the winter also helped).