A few observations:
a 4.75 lb box of borax is $2.99 at walmart, target, and the local grocery store.
SWCGs do not cause staining. However, if you do not watch your pH and allow it to spike that can cause staining (and is usually what DOES precipitate stains.) If you know of two other people near you that have similar problems it is either the salt they are using (not very pure) or it's your water. Even city water can have iron in it.

While solar salt might take a bit longer to dissolve compared to a fine grained pool salt it tends to be very pure and rarely causes staining problems. I have seen 'pool salt' sit on a a pool floor and leave a stain behind. I have never seen solar salt do that. The fact that a pool salt has to include a stain fighter should tell you something. My suspicion that the 'natural stain fighter' is citric acid since that is was is often included in water softener pellets to 'clean' the units. Your expensive pool salt might be no more that ground up water softener pellets.

HEDP does not allow a filter to filter out the metals but this claim is often made. It chemically 'deactivates' the metal ions so they become non reactive for a while. The metal stays in the water until you either replace the water or it drops out of solution as a stain. If the stain is on a filter medium it is possible to change the medium and remove the metal.
The phosphates created by the breakdown of metal sequestrant are exactly the same ones that are 'algae food' but high phosphate levels do not mean you will have algae and are often NOT the limiting factor. Algae also need nitrate. However, there is no easy way to remove nitrate from pool water other than replacement of the water so a profit cannot be made by selling a product as it can with so called phosphate removers.