How to stop copper pipe corrosion (green copper stain in sinks and tubs)
This may not be much help or have anything to do with your installation, but I was told this by a large water heater tank company 30+ years ago. "Very Soft water heated close to and above 140 degrees F. becomes very aggressive and has very detrimental effects on metals it comes in contact with". I do not know if this is true or not, that is what I was told. The next time you speak with someone in the water field, you could ask their opinion and could this be a part of your problem. Also, water with a PH below 7.0 becomes corrosive. I am not a water expert, this just what I have been told.