I've been following this thread since you first posted it because I had similar (I thought) concerns about my apple snail. It wasn't until I read this that I realized that you were talking about the old shell not the new growth.
When I first put Snailio Iglesias in my ten gallon with neon tetras, the pH was below 7. I didn't realize the harm it was doing him until I could see growth on his shell and what looked like cracks forming. After doing a ton of research, I corrected the pH and hardness as much as I safely could (tetras like low pH) by adding a mesh bag of crushed coral to the filter. The pH is now around 7.6. The tetras are fine and Snailio's shell has improved considerably.
Here's a link to my youtube videos
in which you can see the damage to his shell and the growth and repair. (The one called "Snailio Iglesias meets Slois Lane" shows it best.)
One thing you'll note is that the oldest part of his shell is greyish and there is some pitting. The grey parts were there when I got him, but I am not sure about the pitting; it may have happened in my tank because of the pH problem.
Another thing that should stand out is the distinct lines or ridges that run crossways (not the dark lines of coloration that run around the shell in the direction of growth). This is caused by a change in water quality. You can see two of the ridges quite clearly. I think the older one is from when I first got him as that is where the shell scarring starts; the point at which some of the scarring stops is probably from when I increased the pH. The most serious of the scars still continues through that part because it damaged the edge of the shell where the new growth occurs. I think that, with a bit more time, this, too, will end like the other scars did.
All of this is just to say, that if it is the older shell you're concerned about, I doubt that altering the pH or water hardness will fix anything (i.e. regenerate shell in that area), but you can ensure that new growth is solid and that old shell doesn't deteriorate further by adding crushed coral to the filter. A couple of other things I did were to put a piece of cuttlefish bone on the bottom of the tank (no idea if he pays it any attention, but it makes ME feel better!) and to give him spinach or zucchini a couple of times a week (he ignored these the first few times, but I persevered and now he's on them like stink on poop).