Aeration causes the carbon dioxide that is dissolved in the water (as carbonates) to go into the air and this causes the pH to rise with no change in TA. It is the addition of acid that lowers both pH and TA so the procedure for lowering TA combines these two effects by doing aeration and acid addition together.

Aeration --> pH goes Up
Acid --> pH goes Down, TA goes Down
--------------------------------------------
Aeration + Acid --> TA goes Down

So the net result of the above is for the TA to go down which is what you have been seeing over the longer term. In the short term between acid additions, you just see the pH go up.

If you want to continue to use your water feature, you are going to need to live with running at a higher pH level and lower TA level. Since you have high CH, having a lower TA isn't a problem though 70 is probably the right place to be (I wouldn't go below 60). You will have less of a pH rise at higher pH so just keeping the pH at around 7.6 or 7.7 should be easier and isn't a problem in any way. With CYA in the pool, the variation in disinfecting chlorine concentration vs. pH is small. At the lower TA level you are now at, you should find that you add less acid, though you still see a pH rise. If you live with a higher pH target, then you should not only see less quantity of acid, but shouldn't need to add it as frequently. At least you know that with your water feature you have a GREAT aeration device if you ever need to lower your TA! You might try, as an experiment, turning off that feature and seeing if your pH rise slows down significantly.

In your list of measurements you said that the CH reading is hard to determine and sometimes doesn't measure. Are you using the drop-test for CH in the Taylor K-2006 kit or something equivalent? If you find that the endpoint is "fading" so doesn't stay at one color, then you can add some titrant drops to your sample before adding the calcium buffer and indicator. Then just count those drops in your total.

Good luck with the itching investigation. If something works (like heavy cleaning or changing to new swimsuits or changing the sand filter), let us know.

Richard