Nice pool. Have you ruled out organic stains? To test if they are organic stains, you can place a chlorine tablet on the stain for a few minutes and see if it helps lift it some. That is the only thing I use tablets for.

I usually keep no more than 2 around for just this purpose. Sometimes I'll swim down to the stain with a pair of rubber gloves and rub the tablet back and forth against the stain and that seems to work even faster than just letting it sit there.
Do you know how old the pool is and if so, do you know how old the pebble tech finish is as well. Older metal stains are more difficult to get out in my experience. Your mileage may vary. My pool's plaster is from the early 70's and has never been redone. It has quite a bit of pitting in certain areas and of course those areas are more porous and much more susceptible to staining.