If I'm not mistaken, ascorbic acid will lower your PH, but I don't know by how much, so maybe get the PH closer to where it should be before treatment? Since your FC is 0 and will stay there for the AA treatment, you may want to add algaecide (not copper based) to the pool. After the AA removes the stains, you want to introduce chlorine slowly or the stains will redeposit on your pool.

If you don't want these stains to return.... 1. GET RID OF THE NATURE 2 2. Don't let your PH get as high as it apparently has been (8.2 you posted). PH tends to drift up with a SWG...keeping total alkalinity at 80 will slow the PH rise. It worked for me. Also adding 50ppm of borates will dramatically slow the PH rise. MY pool has locked in at 7.6 since dropping TA to 80 and adding borates. 3. GET RID OF THE NATURE 2