Just checking in on this thread; obviously some good progress has been made.

But I have a couple of suggestions.

1. Unless you know of some reason not to do so, remove the Nature2 cartridge, even though it's old. ANY cartridge or filter media is a potential reservoir of accumulated oxidizable 'goo'.

2. For the same reason, clean the filter cartridges. Be sure to do it correctly. Spraying them off (with the hose; NOT with a pressure washer!!) is a good intermediate cleaning, but a full cleaning, which should remove any oxidizable 'goo' that could create chlorine demand takes more. See http://pool9.net/cart-clean/

3. It wasn't clear to me whether you ever inspected light niches or other recessed or hidden areas in the pool. If there are any such recesses, and if it's at all possible, inspect them. It does happen that such locations will fill completely with live 'slime'. Physical cleaning is required; otherwise, the slime will just 'recover' when chlorine levels drop.

Good luck!