In answer to your question, will metals ALWAYS be a problem, I can share some of my experience from the last two years.
I inherited a metals problem last year when my heat exchanger broke, filling my water with copper and iron.
After doing my third vitamin-c treatment, I feel like I finally have a handle on it. Pool looks great after 6 weeks, still.
The key is to be very diligent about following the entire ascorbic acid procedure.
Keep chlorine close to 0 for a few days, PH down to 7.0 or so..Polyquat to keep algae away, Bring chlorine up gradually, and continually use a good sequestrant like Super Sequasol. I was adding 2-4 ounces of sequestrant a day for the first two weeks. I'm down to using just 4oz a week now, and for the most part the stains are gone. Still some staining in the very deep bottom of the pool around the corners, but hardly noticable.
Also, we had the privilege of lots of rain here in Denver this summer, so hopefully it is helping with the dilution of the metals in the pool.