OK,

You are getting confused about things.
1) they tell you turn the pump off when adding acid because they are basing it on the false "Acid Column" theory of lowering alkalinity. It is wrong, doesn't work and can do SERIOUS damage to you liner.
2) To clarify T/A lowering: T/A moves up and down with pH, so you lower it by lowering the pH. The trick is to raise pH without raising T/A, and you do that by aeration--which means making the water splash. It can be as simple as leaving the pool uncovered, or you can point the returns at the surface, or, you can get a return-based fountain. I have one that costs about $20. You unscrew the eyeball and screw this in. This raises pH faster than any other aeration (unless you have a waterfall).
3) You don't want to add any calcium to your pool again.
4) If, after you test your filter with Ben's DE test, and your filter is good, and you add one or two cups of DE, if your flow rate at the return drops and your gauge pressure shoots up, then it's too much DE. Then backwash, and start again, this time adding about 1/3 to 1/2 cup of DE. In 20-30 minutes, if your gauge pressure goes up about 1 lb, you are good.

It's not that hard but the pool store's stock in trade is scaring you and convincing you to buy lots and lots of chemicals you don't need....next they'll try to tell you your phosphate level is too high and try to sell you phosphate remover. 99.99% of the time it's a waste of time and money, won't clear your algae, and distracts you from REAL methods to clear it.

Carl