1. If you have a negative bucket test, it's unlikely you have much metal of any sort (iron, copper, manganese) in your fill water.

2. Between Jacks Magic and Natural Chemistry, I'd go with Jacks Magic. They both do some sleight of hand with their products -- 3 different Jacks Magic products appear to have exactly the same content in the bottle, except for the coloring -- but Natural Chemistry has recently changed the formulation of some other products from marginally effective, to ineffective ingredients, with NO indication of change on the bottle!

3. For using CuLator, the "gallons" figure is purely arbitrary. Also, the packets are not 'used up' unless they change color (except for one unlikely exception), and remain usable so long the packet is not fouled. We recommended getting a larger size, but putting it under a filter sock to keep it from fouling. I'll see if I can post those links below.

4. For clean up, you need a 3 step process: (a) redissolve the metal stain => ascorbic acid (Vitamin C); (b) keep it dissolved => HEDP; (c) remove it from the water => CuLator or filter process. It sounds like, in your case, you may need to use the Vitamin C, and then BRUSH the area -- and that you may need to repeat the process.

If I were you, I'd do the Vitamin C + brushing, everytime the stain reappears, but keep a Culator packet in the skimmer 24/7, for the rest of the season. Remember, the Culator packet can ONLY grab dissolved metals; they have no effect on undissolved metals, like visible stains.

The CuLator product is a special patented plastic material, contained in a bag that goes into the skimmer, that can slowly take metals OUT of your pool water. We think it works, and have some evidence that it does, but not proof yet. (Not likely to be available locally.)
1 ppm Culator @ Amazon
1.5 ppm Culator @ Amazon
4 ppm Culator @ Amazon
Vitamin C Crystals Ascorbic Acid 100% Pure Powder, 3lbs
Poolmaster 16242 Poolmaster Skimmer Basket Liner @ Amazon (to protect CuLator from being damaged by scum)