The problem with the recalled Kent carbon is that the batch of carbon contained unusually high levels of heavy metals. This could definitely be killing corals and fish.
And I could be wrong, but isn't ORP almost useless unless you're running ozone and even then only somewhat usable? ORP is a pretty complex measurement and can be thrown off by many, many different things, magnetic fields, metals, ions, pH, free-radicals, etc. Barometric pressure even probably plays a role. You mention that you just went through a box of salt in the last month. Did you compare the salts even if they are the same brand?
The food probably did spike your nutrients i.e. your ORP rose. You responded with lots of water changes and utilized a toxic carbon. Things started dying. ORP rose again. Maybe the new box of salt has a higher "natural" ORP. So your ORP rose again. And on, and on...
Personally, I'd go back to exactly what I was doing before the disaster. Regular feedings with the regular foods, regular water changes, regular carbon replacement, etc and just let things stablize. I think the Kent carbon and over-reacting caused most of your problems. Hope things get better soon.