Hi,
It's a vinyl liner.

My shocking to 37ppm seems to have made the stains worse

If it made the stain worse I would guess my pH is higher than my test result (due to high chlorine level) and the shocking with adding even more chlorine caused more metal to fall out of solution and make the stains worse.
Correct? If it were organic the shocking would have made it better...

I crushed some 1000mg vitamin c (maybe 12 tablets) in a sock and rubber banded the sock to the end my extendable pole I use for skimming/brushing. I scrubbed the stain (again, it's in the deep end) and no change. I put a chewable tablet (500mg) on the stain and left it there for the night.

I also had a stain from my daughters hairpin that fell out of her hair and caused a rust stain in the shallow end. I scrubbed that with the Vitamin C and no change. I also left a vitamin c tablet on that overnight and will check in the morning if it is gone (or even if the tablet dissolved)

Just for kicks(?) I rubber banded a Mr Clean magic eraser to the end of the pole and scrubbed the yellow spot in the deep end... no change.

Any thoughts on what's going on here???

Thanks from a newbie