Well, I'm full of it (apparently). Your numbers look good and the pool is properly saturated with calcium carbonate -- I assumed that your pH would drop to 7.6 when the temperature of your water went up to around 88F in the summer. That is typical -- all else equal, the pH rises when the temperature drops and lowers again when the temperature comes back up.

So I'm not sure why your spa coping isn't getting affected as much as the area around the pool. Being elevated shouldn't matter unless it was sloped so that water wouldn't settle on the coping. Apparently, the frequent evaporation of salt water and later rehydration cycling is part of what wears some kinds of stone. So I am at a loss to explain the difference unless the spa pH is different during the summer when it gets used. This summer, try testing the pH between the spa and pool. With the jets running and aeration occurring (which is most likely when the spa is used and the kids are splashing water out), the spa may have increased pH and a small spillover is likely to not be enough to mix with the pool water (though you said it shares the same filtration equipment -- does that mean it truly is the same water and that the spa isn't heated, or does the water go into the skimmer and bottom drains of the pool and come out not only in the pool, but also the spa (with possible additional heating before it gets into the spa?).

Anyway, I'm sorry for the problem you are having. If anyone else has limestone coping, perhaps they can share their experiences, especially if theirs is 3 years old or older. In any event, if your coping is replaced, it should probably be replaced by something that is resistant to corrosion generally, including resistant to salt water (and may require annual sealing, similar to my cement hardscape and coping).

Richard