If it lifts with vitamin C, it's iron. I'm glad the stains came up! The iron is now still in the pool, it's just suspended in the water.

Once you pass that overnight test, you can try using some cal-hypo added directly to the skimmer as your chlorine source occasionally. It may help remove some of the suspended iron onto your filter and then it will backwash out. The towel trick will help too. You may find that doing those two things will reduce the iron in the water and allow you to use less sequestrant down the road unless your "utility" refill water is high in iron.