Purple Deposits on pool walls and floor

I have a 17K gallon Sun fiberglass pool that's been installed for 8 years. I use a SWG and sand filter. Also have a Pentair heater installed. Current readings are all in the normal range per the 5-way test strips.

Starting about 4 years ago I developed a problem with purple residue (which I now understand to be Manganese) occasionally coating the side walls and floor of the pool. It has gotten progressively worse for the last 4 years. Now, it's too the point that I can't seem to get it to go away using Metal Sequestering agents such as Metal Magic (expensive at $28.00 per bottle). I've used 4 bottles to no avail. I see an improvement when I use the metal sequester, but it's still there, just less thick on the side walls.

Yesterday I had a local "pool expert" who is our authorized Pentair warranty repair person come out to replace my one year old SWG that had quit working under warranty. I explained my "purple problem" to him and he suggested, almost immediately, that it was my 8 year old pool heater that was causing the problem. He said that they can go bad over time and begin to deteriorate in the interior metal piping, and deposit metals such as manganese and iron into the pool water. He said that since my pool didn't have a backflow preventer, keeping highly chlorinated water from flowing back into the heater, it may have contributed to the problem. He also stated that since I have supplemented chlorine in the skimmer basket when I've had trouble with the SWG in the past, that could have also contributed to the heater breaking down.

So, I need your advice. Does this sound plausible? Will a 8 year old heater deposit metals such as manganese in a pool to the point of turning the water purple. By the way, I also have an iron problem that I have to continually sequester as well.

Also, if I buy a new heater, which one should I get? Should I install a backflow preventor as he suggest?

Thank you so much!

Jonathan