Maybe I overwhelmed everyone with too many questions up front.
Let me break down my thoughts into smaller questions.
First, since it seems I have both soluble and insoluble iron, and a lot of it (dark as tomato soup when unfiltered), I assume I can't replace the whole greensand/pot perm setup with a cartridge filter, 1 or 5 micron? Like Purenex or Culligan makes? I'm not interested in getting 'drinkable' water, just clear to the eye for baths. The cartridges seem cheap, I wouldn't have an issue with changing the cartridge every 200 gallons or so.